Improve Syndication DateTime Parsing (#25309)

* Improve Syndication DateTime Parsing.

Fix #25156

* Enable custom parser.

* Enabled StringParser.

Added tests for custom DateTimeParser and StringParser.

* Enable Custom UriParser.

Added tests for custom UriParser.

* Removed StringParser.

* Addressed CR Feedback.

* Unform LocalNames Passed to UriParser.

10 files changed, 554 insertions, 270 deletions

diff --git a/src/System.ServiceModel.Syndication/ref/System.ServiceModel.Syndication.cs b/src/System.ServiceModel.Syndication/ref/System.ServiceModel.Syndication.cs
index e463c141e8..7103feeaa0 100644
--- a/src/System.ServiceModel.Syndication/ref/System.ServiceModel.Syndication.cs
+++ b/src/System.ServiceModel.Syndication/ref/System.ServiceModel.Syndication.cs
@@ -391,7 +391,9 @@ namespace System.ServiceModel.Syndication
     {
         protected SyndicationFeedFormatter() { }
         protected SyndicationFeedFormatter(System.ServiceModel.Syndication.SyndicationFeed feedToWrite) { }
+        public System.Func<string, string, string, System.DateTimeOffset> DateTimeParser { get { throw null; } set { } }
         public System.ServiceModel.Syndication.SyndicationFeed Feed { get { throw null; } }
+        public System.Func<string, System.UriKind, string, string, System.Uri> UriParser { get { throw null; } set { } }
         public abstract string Version { get; }
         public abstract bool CanRead(System.Xml.XmlReader reader);
         protected internal static System.ServiceModel.Syndication.SyndicationCategory CreateCategory(System.ServiceModel.Syndication.SyndicationFeed feed) { throw null; }
@@ -616,4 +618,4 @@ namespace System.ServiceModel.Syndication
         public TContent ReadContent<TContent>(System.Xml.Serialization.XmlSerializer serializer) { throw null; }
         protected override void WriteContentsTo(System.Xml.XmlWriter writer) { }
     }
-}
\ No newline at end of file
+}
diff --git a/src/System.ServiceModel.Syndication/src/System/ServiceModel/Syndication/Atom10FeedFormatter.cs b/src/System.ServiceModel.Syndication/src/System/ServiceModel/Syndication/Atom10FeedFormatter.cs
index a01d6d074d..db52a42c56 100644
--- a/src/System.ServiceModel.Syndication/src/System/ServiceModel/Syndication/Atom10FeedFormatter.cs
+++ b/src/System.ServiceModel.Syndication/src/System/ServiceModel/Syndication/Atom10FeedFormatter.cs
@@ -21,7 +21,7 @@ namespace System.ServiceModel.Syndication
     [XmlRoot(ElementName = Atom10Constants.FeedTag, Namespace = Atom10Constants.Atom10Namespace)]
     public class Atom10FeedFormatter : SyndicationFeedFormatter, IXmlSerializable
     {
-        internal static readonly TimeSpan zeroOffset = new TimeSpan(0, 0, 0);
+        internal static readonly TimeSpan ZeroOffset = new TimeSpan(0, 0, 0);
         internal const string XmlNs = "http://www.w3.org/XML/1998/namespace";
         internal const string XmlNsNs = "http://www.w3.org/2000/xmlns/";
         private static readonly XmlQualifiedName s_atom10Href = new XmlQualifiedName(Atom10Constants.HrefTag, string.Empty);
@@ -71,6 +71,11 @@ namespace System.ServiceModel.Syndication
             _feedType = feedToWrite.GetType();
         }
 
+        internal override Func<string, string, string, DateTimeOffset> GetDefaultDateTimeParser()
+        {
+            return DateTimeHelper.DefaultAtom10DateTimeParser;
+        }
+
         public bool PreserveAttributeExtensions
         {
             get { return _preserveAttributeExtensions; }
@@ -304,7 +309,7 @@ namespace System.ServiceModel.Syndication
             }
             else if (reader.IsStartElement(Atom10Constants.LogoTag, Atom10Constants.Atom10Namespace))
             {
-                result.ImageUrl = new Uri(reader.ReadElementString(), UriKind.RelativeOrAbsolute);
+                result.ImageUrl = UriParser(reader.ReadElementString(), UriKind.RelativeOrAbsolute, Atom10Constants.LogoTag, Atom10Constants.Atom10Namespace);
             }
             else if (reader.IsStartElement(Atom10Constants.RightsTag, Atom10Constants.Atom10Namespace))
             {
@@ -321,7 +326,16 @@ namespace System.ServiceModel.Syndication
             else if (reader.IsStartElement(Atom10Constants.UpdatedTag, Atom10Constants.Atom10Namespace))
             {
                 reader.ReadStartElement();
-                result.LastUpdatedTime = DateFromString(reader.ReadString(), reader);
+                string dtoString = reader.ReadString();
+                try
+                {
+                    result.LastUpdatedTime = DateFromString(dtoString, reader);
+                }
+                catch (XmlException e)
+                {
+                    result.LastUpdatedTimeException = e;
+                }
+
                 reader.ReadEndElement();
             }
             else
@@ -360,7 +374,16 @@ namespace System.ServiceModel.Syndication
             else if (reader.IsStartElement(Atom10Constants.PublishedTag, Atom10Constants.Atom10Namespace))
             {
                 reader.ReadStartElement();
-                result.PublishDate = DateFromString(reader.ReadString(), reader);
+                string dtoString = reader.ReadString();
+                try
+                {
+                    result.PublishDate = DateFromString(dtoString, reader);
+                }
+                catch (XmlException e)
+                {
+                    result.PublishDateException = e;
+                }
+
                 reader.ReadEndElement();
             }
             else if (reader.IsStartElement(Atom10Constants.RightsTag, Atom10Constants.Atom10Namespace))
@@ -384,7 +407,16 @@ namespace System.ServiceModel.Syndication
             else if (reader.IsStartElement(Atom10Constants.UpdatedTag, Atom10Constants.Atom10Namespace))
             {
                 reader.ReadStartElement();
-                result.LastUpdatedTime = DateFromString(reader.ReadString(), reader);
+                string dtoString = reader.ReadString();
+                try
+                {
+                    result.LastUpdatedTime = DateFromString(dtoString, reader);
+                }
+                catch (XmlException e)
+                {
+                    result.LastUpdatedTimeException = e;
+                }
+
                 reader.ReadEndElement();
             }
             else
@@ -624,6 +656,8 @@ namespace System.ServiceModel.Syndication
                 }
             }
             reader.MoveToElement();
+            string localName = reader.LocalName;
+            string nameSpace = reader.NamespaceURI;
             string val = (kind == TextSyndicationContentKind.XHtml) ? reader.ReadInnerXml() : reader.ReadElementString();
             TextSyndicationContent result = new TextSyndicationContent(val, kind);
             if (attrs != null)
@@ -641,7 +675,7 @@ namespace System.ServiceModel.Syndication
 
         private string AsString(DateTimeOffset dateTime)
         {
-            if (dateTime.Offset == zeroOffset)
+            if (dateTime.Offset == ZeroOffset)
             {
                 return dateTime.ToUniversalTime().ToString(Rfc3339UTCDateTimeFormat, CultureInfo.InvariantCulture);
             }
@@ -651,44 +685,6 @@ namespace System.ServiceModel.Syndication
             }
         }
 
-        private DateTimeOffset DateFromString(string dateTimeString, XmlReader reader)
-        {
-            dateTimeString = dateTimeString.Trim();
-            if (dateTimeString.Length < 20)
-            {
-                throw DiagnosticUtility.ExceptionUtility.ThrowHelperError(
-                    new XmlException(FeedUtils.AddLineInfo(reader,
-                    SR.ErrorParsingDateTime)));
-            }
-            if (dateTimeString[19] == '.')
-            {
-                // remove any fractional seconds, we choose to ignore them
-                int i = 20;
-                while (dateTimeString.Length > i && char.IsDigit(dateTimeString[i]))
-                {
-                    ++i;
-                }
-                dateTimeString = dateTimeString.Substring(0, 19) + dateTimeString.Substring(i);
-            }
-            DateTimeOffset localTime;
-            if (DateTimeOffset.TryParseExact(dateTimeString, Rfc3339LocalDateTimeFormat,
-                CultureInfo.InvariantCulture.DateTimeFormat,
-                DateTimeStyles.None, out localTime))
-            {
-                return localTime;
-            }
-            DateTimeOffset utcTime;
-            if (DateTimeOffset.TryParseExact(dateTimeString, Rfc3339UTCDateTimeFormat,
-                CultureInfo.InvariantCulture.DateTimeFormat,
-                DateTimeStyles.AssumeUniversal | DateTimeStyles.AdjustToUniversal, out utcTime))
-            {
-                return utcTime;
-            }
-            throw DiagnosticUtility.ExceptionUtility.ThrowHelperError(
-                new XmlException(FeedUtils.AddLineInfo(reader,
-                SR.ErrorParsingDateTime)));
-        }
-
         private void ReadCategory(XmlReader reader, SyndicationCategory category)
         {
             ReadCategory(reader, category, this.Version, this.PreserveAttributeExtensions, this.PreserveElementExtensions, _maxExtensionSize);
@@ -732,7 +728,7 @@ namespace System.ServiceModel.Syndication
 
             if (!string.IsNullOrEmpty(src))
             {
-                result = new UrlSyndicationContent(new Uri(src, UriKind.RelativeOrAbsolute), type);
+                result = new UrlSyndicationContent(UriParser(src, UriKind.RelativeOrAbsolute, Atom10Constants.ContentTag, Atom10Constants.Atom10Namespace), type);
                 bool isEmpty = reader.IsEmptyElement;
                 if (reader.HasAttributes)
                 {
@@ -1085,7 +1081,7 @@ namespace System.ServiceModel.Syndication
             link.MediaType = mediaType;
             link.RelationshipType = relationship;
             link.Title = title;
-            link.Uri = (val != null) ? new Uri(val, UriKind.RelativeOrAbsolute) : null;
+            link.Uri = (val != null) ? UriParser(val, UriKind.RelativeOrAbsolute, Atom10Constants.LinkTag, Atom10Constants.Atom10Namespace) : null;
         }
 
         private SyndicationLink ReadLinkFrom(XmlReader reader, SyndicationFeed feed)
diff --git a/src/System.ServiceModel.Syndication/src/System/ServiceModel/Syndication/DateTimeHelper.cs b/src/System.ServiceModel.Syndication/src/System/ServiceModel/Syndication/DateTimeHelper.cs
new file mode 100644
index 0000000000..9ca15bf074
--- /dev/null
+++ b/src/System.ServiceModel.Syndication/src/System/ServiceModel/Syndication/DateTimeHelper.cs
@@ -0,0 +1,217 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System.Globalization;
+using System.Text;
+
+namespace System.ServiceModel.Syndication
+{
+    internal static class DateTimeHelper
+    {
+        private const string Rfc3339DateTimeFormat = "yyyy-MM-ddTHH:mm:ssK";
+
+        public static DateTimeOffset DefaultRss20DateTimeParser(string dateTimeString, string localName, string ns)
+        {
+            DateTimeOffset dto;
+
+            // First check if DateTimeOffset default parsing can parse the date
+            if (DateTimeOffset.TryParse(dateTimeString, out dto))
+            {
+                return dto;
+            }
+
+            // RSS specifies RFC822
+            if (Rfc822DateTimeParser(dateTimeString, out dto))
+            {
+                return dto;
+            }
+
+            // Event though RCS3339 is for Atom, someone might be using this for RSS
+            if (Rfc3339DateTimeParser(dateTimeString, out dto))
+            {
+                return dto;
+            }
+
+            // Unable to parse - using a default date;
+            throw new FormatException(SR.ErrorParsingDateTime);
+        }
+
+        public static DateTimeOffset DefaultAtom10DateTimeParser(string dateTimeString, string localName, string ns)
+        {
+            if (Rfc3339DateTimeParser(dateTimeString, out DateTimeOffset dto))
+            {
+                return dto;
+            }
+
+            throw new FormatException(SR.ErrorParsingDateTime);
+        }
+
+        private static bool Rfc3339DateTimeParser(string dateTimeString, out DateTimeOffset dto)
+        {
+            dateTimeString = dateTimeString.Trim();
+            if (dateTimeString.Length < 20)
+            {
+                return false;
+            }
+
+            if (dateTimeString[19] == '.')
+            {
+                // remove any fractional seconds, we choose to ignore them
+                int i = 20;
+                while (dateTimeString.Length > i && char.IsDigit(dateTimeString[i]))
+                {
+                    ++i;
+                }
+
+                dateTimeString = dateTimeString.Substring(0, 19) + dateTimeString.Substring(i);
+            }
+
+            return DateTimeOffset.TryParseExact(dateTimeString, Rfc3339DateTimeFormat, CultureInfo.InvariantCulture.DateTimeFormat, DateTimeStyles.None, out dto);
+        }
+
+        private static bool Rfc822DateTimeParser(string dateTimeString, out DateTimeOffset dto)
+        {
+            StringBuilder dateTimeStringBuilder = new StringBuilder(dateTimeString.Trim());
+            if (dateTimeStringBuilder.Length < 18)
+            {
+                return false;
+            }
+
+            int timeZoneStartIndex;
+            for (timeZoneStartIndex = dateTimeStringBuilder.Length - 1; dateTimeStringBuilder[timeZoneStartIndex] != ' '; timeZoneStartIndex--)
+                ;
+            timeZoneStartIndex++;
+
+            int timeZoneLength = dateTimeStringBuilder.Length - timeZoneStartIndex;
+            string timeZoneSuffix = dateTimeStringBuilder.ToString(timeZoneStartIndex, timeZoneLength);
+            dateTimeStringBuilder.Remove(timeZoneStartIndex, timeZoneLength);
+            bool isUtc;
+            dateTimeStringBuilder.Append(NormalizeTimeZone(timeZoneSuffix, out isUtc));
+            string wellFormattedString = dateTimeStringBuilder.ToString();
+
+            DateTimeOffset theTime;
+            string[] parseFormat =
+            {
+                "ddd, dd MMMM yyyy HH:mm:ss zzz",
+                "dd MMMM yyyy HH:mm:ss zzz",
+                "ddd, dd MMM yyyy HH:mm:ss zzz",
+                "dd MMM yyyy HH:mm:ss zzz",
+
+                "ddd, dd MMMM yyyy HH:mm zzz",
+                "dd MMMM yyyy HH:mm zzz",
+                "ddd, dd MMM yyyy HH:mm zzz",
+                "dd MMM yyyy HH:mm zzz",
+
+                // The original RFC822 spec listed 2 digit years. RFC1123 updated the format to include 4 digit years and states that you should use 4 digits.
+                // Technically RSS2.0 specifies RFC822 but it's presumed that RFC1123 will be used as we're now past Y2K and everyone knows better. The 4 digit
+                // formats are listed first for performance reasons as it's presumed they will be more likely to match first.
+                "ddd, dd MMMM yy HH:mm:ss zzz",
+                "dd MMMM yyyy HH:mm:ss zzz",
+                "ddd, dd MMM yy HH:mm:ss zzz",
+                "dd MMM yyyy HH:mm:ss zzz",
+
+                "ddd, dd MMMM yy HH:mm zzz",
+                "dd MMMM yyyy HH:mm zzz",
+                "ddd, dd MMM yy HH:mm zzz",
+                "dd MMM yyyy HH:mm zzz"
+            };
+
+            if (DateTimeOffset.TryParseExact(wellFormattedString, parseFormat,
+                CultureInfo.InvariantCulture.DateTimeFormat,
+                (isUtc ? DateTimeStyles.AdjustToUniversal : DateTimeStyles.None), out theTime))
+            {
+                dto = theTime;
+                return true;
+            }
+
+            return false;
+        }
+
+        private static string NormalizeTimeZone(string rfc822TimeZone, out bool isUtc)
+        {
+            isUtc = false;
+            // return a string in "-08:00" format
+            if (rfc822TimeZone[0] == '+' || rfc822TimeZone[0] == '-')
+            {
+                // the time zone is supposed to be 4 digits but some feeds omit the initial 0
+                StringBuilder result = new StringBuilder(rfc822TimeZone);
+                if (result.Length == 4)
+                {
+                    // the timezone is +/-HMM. Convert to +/-HHMM
+                    result.Insert(1, '0');
+                }
+                result.Insert(3, ':');
+                return result.ToString();
+            }
+            switch (rfc822TimeZone)
+            {
+                case "UT":
+                case "Z":
+                    isUtc = true;
+                    return "-00:00";
+                case "GMT":
+                    return "-00:00";
+                case "A":
+                    return "-01:00";
+                case "B":
+                    return "-02:00";
+                case "C":
+                    return "-03:00";
+                case "D":
+                case "EDT":
+                    return "-04:00";
+                case "E":
+                case "EST":
+                case "CDT":
+                    return "-05:00";
+                case "F":
+                case "CST":
+                case "MDT":
+                    return "-06:00";
+                case "G":
+                case "MST":
+                case "PDT":
+                    return "-07:00";
+                case "H":
+                case "PST":
+                    return "-08:00";
+                case "I":
+                    return "-09:00";
+                case "K":
+                    return "-10:00";
+                case "L":
+                    return "-11:00";
+                case "M":
+                    return "-12:00";
+                case "N":
+                    return "+01:00";
+                case "O":
+                    return "+02:00";
+                case "P":
+                    return "+03:00";
+                case "Q":
+                    return "+04:00";
+                case "R":
+                    return "+05:00";
+                case "S":
+                    return "+06:00";
+                case "T":
+                    return "+07:00";
+                case "U":
+                    return "+08:00";
+                case "V":
+                    return "+09:00";
+                case "W":
+                    return "+10:00";
+                case "X":
+                    return "+11:00";
+                case "Y":
+                    return "+12:00";
+                default:
+                    return "";
+            }
+        }
+
+    }
+}
diff --git a/src/System.ServiceModel.Syndication/src/System/ServiceModel/Syndication/Rss20FeedFormatter.cs b/src/System.ServiceModel.Syndication/src/System/ServiceModel/Syndication/Rss20FeedFormatter.cs
index de97d03a9d..409c24b87d 100644
--- a/src/System.ServiceModel.Syndication/src/System/ServiceModel/Syndication/Rss20FeedFormatter.cs
+++ b/src/System.ServiceModel.Syndication/src/System/ServiceModel/Syndication/Rss20FeedFormatter.cs
@@ -78,6 +78,11 @@ namespace System.ServiceModel.Syndication
             _feedType = feedToWrite.GetType();
         }
 
+        internal override Func<string, string, string, DateTimeOffset> GetDefaultDateTimeParser()
+        {
+            return DateTimeHelper.DefaultRss20DateTimeParser;
+        }
+
         public bool PreserveAttributeExtensions
         {
             get { return _preserveAttributeExtensions; }
@@ -254,211 +259,10 @@ namespace System.ServiceModel.Syndication
                 this.WriteItem(writer, item, feedBaseUri);
             }
         }
-
-        private static DateTimeOffset DateFromString(string dateTimeString, XmlReader reader)
-        {
-            StringBuilder dateTimeStringBuilder = new StringBuilder(dateTimeString.Trim());
-            if (dateTimeStringBuilder.Length < 18)
-            {
-                throw DiagnosticUtility.ExceptionUtility.ThrowHelperError(
-                    new XmlException(FeedUtils.AddLineInfo(reader,
-                    SR.ErrorParsingDateTime)));
-            }
-            if (dateTimeStringBuilder[3] == ',')
-            {
-                // There is a leading (e.g.) "Tue, ", strip it off
-                dateTimeStringBuilder.Remove(0, 4);
-                // There's supposed to be a space here but some implementations dont have one
-                RemoveExtraWhiteSpaceAtStart(dateTimeStringBuilder);
-            }
-            ReplaceMultipleWhiteSpaceWithSingleWhiteSpace(dateTimeStringBuilder);
-            if (char.IsDigit(dateTimeStringBuilder[1]))
-            {
-                // two-digit day, we are good
-            }
-            else
-            {
-                dateTimeStringBuilder.Insert(0, '0');
-            }
-            if (dateTimeStringBuilder.Length < 19)
-            {
-                throw DiagnosticUtility.ExceptionUtility.ThrowHelperError(
-                    new XmlException(FeedUtils.AddLineInfo(reader,
-                    SR.ErrorParsingDateTime)));
-            }
-            bool thereAreSeconds = (dateTimeStringBuilder[17] == ':');
-            int timeZoneStartIndex;
-            if (thereAreSeconds)
-            {
-                timeZoneStartIndex = 21;
-            }
-            else
-            {
-                timeZoneStartIndex = 18;
-            }
-            string timeZoneSuffix = dateTimeStringBuilder.ToString().Substring(timeZoneStartIndex);
-            dateTimeStringBuilder.Remove(timeZoneStartIndex, dateTimeStringBuilder.Length - timeZoneStartIndex);
-            bool isUtc;
-            dateTimeStringBuilder.Append(NormalizeTimeZone(timeZoneSuffix, out isUtc));
-            string wellFormattedString = dateTimeStringBuilder.ToString();
-
-            DateTimeOffset theTime;
-            string parseFormat;
-            if (thereAreSeconds)
-            {
-                parseFormat = "dd MMM yyyy HH:mm:ss zzz";
-            }
-            else
-            {
-                parseFormat = "dd MMM yyyy HH:mm zzz";
-            }
-            if (DateTimeOffset.TryParseExact(wellFormattedString, parseFormat,
-                CultureInfo.InvariantCulture.DateTimeFormat,
-                (isUtc ? DateTimeStyles.AdjustToUniversal : DateTimeStyles.None), out theTime))
-            {
-                return theTime;
-            }
-            throw DiagnosticUtility.ExceptionUtility.ThrowHelperError(
-                new XmlException(FeedUtils.AddLineInfo(reader,
-                SR.ErrorParsingDateTime)));
-        }
-
-        private static string NormalizeTimeZone(string rfc822TimeZone, out bool isUtc)
-        {
-            isUtc = false;
-            // return a string in "-08:00" format
-            if (rfc822TimeZone[0] == '+' || rfc822TimeZone[0] == '-')
-            {
-                // the time zone is supposed to be 4 digits but some feeds omit the initial 0
-                StringBuilder result = new StringBuilder(rfc822TimeZone);
-                if (result.Length == 4)
-                {
-                    // the timezone is +/-HMM. Convert to +/-HHMM
-                    result.Insert(1, '0');
-                }
-                result.Insert(3, ':');
-                return result.ToString();
-            }
-            switch (rfc822TimeZone)
-            {
-                case "UT":
-                case "Z":
-                    isUtc = true;
-                    return "-00:00";
-                case "GMT":
-                    return "-00:00";
-                case "A":
-                    return "-01:00";
-                case "B":
-                    return "-02:00";
-                case "C":
-                    return "-03:00";
-                case "D":
-                case "EDT":
-                    return "-04:00";
-                case "E":
-                case "EST":
-                case "CDT":
-                    return "-05:00";
-                case "F":
-                case "CST":
-                case "MDT":
-                    return "-06:00";
-                case "G":
-                case "MST":
-                case "PDT":
-                    return "-07:00";
-                case "H":
-                case "PST":
-                    return "-08:00";
-                case "I":
-                    return "-09:00";
-                case "K":
-                    return "-10:00";
-                case "L":
-                    return "-11:00";
-                case "M":
-                    return "-12:00";
-                case "N":
-                    return "+01:00";
-                case "O":
-                    return "+02:00";
-                case "P":
-                    return "+03:00";
-                case "Q":
-                    return "+04:00";
-                case "R":
-                    return "+05:00";
-                case "S":
-                    return "+06:00";
-                case "T":
-                    return "+07:00";
-                case "U":
-                    return "+08:00";
-                case "V":
-                    return "+09:00";
-                case "W":
-                    return "+10:00";
-                case "X":
-                    return "+11:00";
-                case "Y":
-                    return "+12:00";
-                default:
-                    return "";
-            }
-        }
-
-        private static void RemoveExtraWhiteSpaceAtStart(StringBuilder stringBuilder)
-        {
-            int i = 0;
-            while (i < stringBuilder.Length)
-            {
-                if (!char.IsWhiteSpace(stringBuilder[i]))
-                {
-                    break;
-                }
-                ++i;
-            }
-            if (i > 0)
-            {
-                stringBuilder.Remove(0, i);
-            }
-        }
-
-        private static void ReplaceMultipleWhiteSpaceWithSingleWhiteSpace(StringBuilder builder)
-        {
-            int index = 0;
-            int whiteSpaceStart = -1;
-            while (index < builder.Length)
-            {
-                if (char.IsWhiteSpace(builder[index]))
-                {
-                    if (whiteSpaceStart < 0)
-                    {
-                        whiteSpaceStart = index;
-                        // normalize all white spaces to be ' ' so that the date time parsing works
-                        builder[index] = ' ';
-                    }
-                }
-                else if (whiteSpaceStart >= 0)
-                {
-                    if (index > whiteSpaceStart + 1)
-                    {
-                        // there are at least 2 spaces... replace by 1
-                        builder.Remove(whiteSpaceStart, index - whiteSpaceStart - 1);
-                        index = whiteSpaceStart + 1;
-                    }
-                    whiteSpaceStart = -1;
-                }
-                ++index;
-            }
-            // we have already trimmed the start and end so there cannot be a trail of white spaces in the end
-            Debug.Assert(builder.Length == 0 || builder[builder.Length - 1] != ' ', "The string builder doesnt end in a white space");
-        }
-
+        
         private string AsString(DateTimeOffset dateTime)
         {
-            if (dateTime.Offset == Atom10FeedFormatter.zeroOffset)
+            if (dateTime.Offset == Atom10FeedFormatter.ZeroOffset)
             {
                 return dateTime.ToUniversalTime().ToString(Rfc822OutputUtcDateTimeFormat, CultureInfo.InvariantCulture);
             }
@@ -497,8 +301,9 @@ namespace System.ServiceModel.Syndication
                     }
                 }
             }
+
             string uri = reader.ReadElementString();
-            link.Uri = new Uri(uri, UriKind.RelativeOrAbsolute);
+            link.Uri = UriParser(uri, UriKind.RelativeOrAbsolute, Rss20Constants.LinkTag, Rss20Constants.Rss20Namespace);
             return link;
         }
 
@@ -601,6 +406,8 @@ namespace System.ServiceModel.Syndication
                 if (!isEmpty)
                 {
                     string fallbackAlternateLink = null;
+                    string fallbackAlternateLinkLocalName = null;
+                    string fallbackAlternateLinkNamespace = null;
                     XmlDictionaryWriter extWriter = null;
                     bool readAlternateLink = false;
                     try
@@ -641,10 +448,13 @@ namespace System.ServiceModel.Syndication
                                 {
                                     isPermalink = false;
                                 }
+
                                 result.Id = reader.ReadElementString();
                                 if (isPermalink)
                                 {
                                     fallbackAlternateLink = result.Id;
+                                    fallbackAlternateLinkLocalName = Rss20Constants.GuidTag;
+                                    fallbackAlternateLinkNamespace = Rss20Constants.Rss20Namespace;
                                 }
                             }
                             else if (reader.IsStartElement(Rss20Constants.PubDateTag, Rss20Constants.Rss20Namespace))
@@ -656,7 +466,14 @@ namespace System.ServiceModel.Syndication
                                     string str = reader.ReadString();
                                     if (!string.IsNullOrEmpty(str))
                                     {
-                                        result.PublishDate = DateFromString(str, reader);
+                                        try
+                                        {
+                                            result.PublishDate = DateFromString(str, reader);
+                                        }
+                                        catch (XmlException e)
+                                        {
+                                            result.PublishDateException = e;
+                                        }
                                     }
                                     reader.ReadEndElement();
                                 }
@@ -677,7 +494,7 @@ namespace System.ServiceModel.Syndication
                                         string val = reader.Value;
                                         if (name == Rss20Constants.UrlTag && ns == Rss20Constants.Rss20Namespace)
                                         {
-                                            feed.Links.Add(SyndicationLink.CreateSelfLink(new Uri(val, UriKind.RelativeOrAbsolute)));
+                                            feed.Links.Add(SyndicationLink.CreateSelfLink(UriParser(val, UriKind.RelativeOrAbsolute, Rss20Constants.UrlTag, Rss20Constants.Rss20Namespace)));
                                         }
                                         else if (!FeedUtils.IsXmlns(name, ns))
                                         {
@@ -692,6 +509,7 @@ namespace System.ServiceModel.Syndication
                                         }
                                     }
                                 }
+
                                 string feedTitle = reader.ReadElementString();
                                 feed.Title = new TextSyndicationContent(feedTitle);
                                 result.SourceFeed = feed;
@@ -729,7 +547,7 @@ namespace System.ServiceModel.Syndication
                     reader.ReadEndElement(); // item
                     if (!readAlternateLink && fallbackAlternateLink != null)
                     {
-                        result.Links.Add(SyndicationLink.CreateAlternateLink(new Uri(fallbackAlternateLink, UriKind.RelativeOrAbsolute)));
+                        result.Links.Add(SyndicationLink.CreateAlternateLink(UriParser(fallbackAlternateLink, UriKind.RelativeOrAbsolute, fallbackAlternateLinkLocalName, fallbackAlternateLinkNamespace)));
                         readAlternateLink = true;
                     }
 
@@ -775,7 +593,7 @@ namespace System.ServiceModel.Syndication
                     string val = reader.Value;
                     if (name == Rss20Constants.UrlTag && ns == Rss20Constants.Rss20Namespace)
                     {
-                        link.Uri = new Uri(val, UriKind.RelativeOrAbsolute);
+                        link.Uri = UriParser(val, UriKind.RelativeOrAbsolute, Rss20Constants.EnclosureTag, Rss20Constants.Rss20Namespace);
                     }
                     else if (name == Rss20Constants.TypeTag && ns == Rss20Constants.Rss20Namespace)
                     {
@@ -861,7 +679,11 @@ namespace System.ServiceModel.Syndication
             try
             {
                 string baseUri = null;
+                string baseUriLocalName = null;
+                string baseUriNamespace = null;
                 reader.MoveToContent();
+                string elementLocalName = reader.LocalName;
+                string elementNamespace = reader.NamespaceURI;
                 string version = reader.GetAttribute(Rss20Constants.VersionTag, Rss20Constants.Rss20Namespace);
                 if (version != Rss20Constants.Version)
                 {
@@ -873,10 +695,14 @@ namespace System.ServiceModel.Syndication
                     if (!string.IsNullOrEmpty(tmp))
                     {
                         baseUri = tmp;
+                        baseUriLocalName = elementLocalName;
+                        baseUriNamespace = elementNamespace;
                     }
                 }
                 reader.ReadStartElement();
                 reader.MoveToContent();
+                elementLocalName = reader.LocalName;
+                elementNamespace = reader.NamespaceURI;
                 if (reader.HasAttributes)
                 {
                     while (reader.MoveToNextAttribute())
@@ -886,6 +712,8 @@ namespace System.ServiceModel.Syndication
                         if (name == "base" && ns == Atom10FeedFormatter.XmlNs)
                         {
                             baseUri = reader.Value;
+                            baseUriLocalName = elementLocalName;
+                            baseUriNamespace = elementNamespace;
                             continue;
                         }
                         if (FeedUtils.IsXmlns(name, ns) || FeedUtils.IsXmlSchemaType(name, ns))
@@ -906,10 +734,12 @@ namespace System.ServiceModel.Syndication
                         }
                     }
                 }
+
                 if (!string.IsNullOrEmpty(baseUri))
                 {
-                    result.BaseUri = new Uri(baseUri, UriKind.RelativeOrAbsolute);
+                    result.BaseUri = UriParser(baseUri, UriKind.RelativeOrAbsolute, baseUriLocalName, baseUriNamespace);
                 }
+
                 bool areAllItemsRead = true;
                 reader.ReadStartElement(Rss20Constants.ChannelTag, Rss20Constants.Rss20Namespace);
 
@@ -954,7 +784,14 @@ namespace System.ServiceModel.Syndication
                                 string str = reader.ReadString();
                                 if (!string.IsNullOrEmpty(str))
                                 {
-                                    result.LastUpdatedTime = DateFromString(str, reader);
+                                    try
+                                    {
+                                        result.LastUpdatedTime = DateFromString(str, reader);
+                                    }
+                                    catch (XmlException e)
+                                    {
+                                        result.LastUpdatedTimeException = e;
+                                    }
                                 }
                                 reader.ReadEndElement();
                             }
@@ -974,7 +811,7 @@ namespace System.ServiceModel.Syndication
                             {
                                 if (reader.IsStartElement(Rss20Constants.UrlTag, Rss20Constants.Rss20Namespace))
                                 {
-                                    result.ImageUrl = new Uri(reader.ReadElementString(), UriKind.RelativeOrAbsolute);
+                                    result.ImageUrl = UriParser(reader.ReadElementString(), UriKind.RelativeOrAbsolute, Rss20Constants.UrlTag, Rss20Constants.Rss20Namespace);
                                 }
                                 else
                                 {
diff --git a/src/System.ServiceModel.Syndication/src/System/ServiceModel/Syndication/SyndicationFeed.cs b/src/System.ServiceModel.Syndication/src/System/ServiceModel/Syndication/SyndicationFeed.cs
index 6df7450bd7..621c98994f 100644
--- a/src/System.ServiceModel.Syndication/src/System/ServiceModel/Syndication/SyndicationFeed.cs
+++ b/src/System.ServiceModel.Syndication/src/System/ServiceModel/Syndication/SyndicationFeed.cs
@@ -227,10 +227,24 @@ namespace System.ServiceModel.Syndication
             set { _language = value; }
         }
 
+        internal Exception LastUpdatedTimeException { get; set; }
+
         public DateTimeOffset LastUpdatedTime
         {
-            get { return _lastUpdatedTime; }
-            set { _lastUpdatedTime = value; }
+            get
+            {
+                if (LastUpdatedTimeException != null)
+                {
+                    throw LastUpdatedTimeException;
+                }
+
+                return _lastUpdatedTime;
+            }
+            set
+            {
+                LastUpdatedTimeException = null;
+                _lastUpdatedTime = value;
+            }
         }
 
         public Collection<SyndicationLink> Links
diff --git a/src/System.ServiceModel.Syndication/src/System/ServiceModel/Syndication/SyndicationFeedFormatter.cs b/src/System.ServiceModel.Syndication/src/System/ServiceModel/Syndication/SyndicationFeedFormatter.cs
index fa89a8493b..750563c606 100644
--- a/src/System.ServiceModel.Syndication/src/System/ServiceModel/Syndication/SyndicationFeedFormatter.cs
+++ b/src/System.ServiceModel.Syndication/src/System/ServiceModel/Syndication/SyndicationFeedFormatter.cs
@@ -7,11 +7,9 @@ namespace System.ServiceModel.Syndication
     using System;
     using System.Diagnostics;
     using System.Globalization;
-    using System.Runtime;
     using System.Runtime.Serialization;
     using System.Xml;
     using DiagnosticUtility = System.ServiceModel.DiagnosticUtility;
-    using System.Runtime.CompilerServices;
 
     [DataContract]
     public abstract class SyndicationFeedFormatter
@@ -21,6 +19,7 @@ namespace System.ServiceModel.Syndication
         protected SyndicationFeedFormatter()
         {
             _feed = null;
+            DateTimeParser = GetDefaultDateTimeParser();
         }
 
         protected SyndicationFeedFormatter(SyndicationFeed feedToWrite)
@@ -30,6 +29,7 @@ namespace System.ServiceModel.Syndication
                 throw DiagnosticUtility.ExceptionUtility.ThrowHelperArgumentNull("feedToWrite");
             }
             _feed = feedToWrite;
+            DateTimeParser = GetDefaultDateTimeParser();
         }
 
         public SyndicationFeed Feed
@@ -40,6 +40,21 @@ namespace System.ServiceModel.Syndication
             }
         }
 
+        public Func<string, UriKind, string, string, Uri> UriParser { get; set; } = DefaultUriParser;
+
+        // Different DateTimeParsers are needed for Atom and Rss so can't set inline
+        public Func<string, string, string, DateTimeOffset> DateTimeParser { get; set; }
+
+        internal virtual Func<string, string, string, DateTimeOffset> GetDefaultDateTimeParser()
+        {
+            return NotImplementedDateTimeParser;
+        }
+
+        private DateTimeOffset NotImplementedDateTimeParser(string dtoString, string localName, string ns)
+        {
+            throw new NotImplementedException();
+        }
+
         public abstract string Version
         { get; }
 
@@ -376,6 +391,24 @@ namespace System.ServiceModel.Syndication
             _feed = feed;
         }
 
+        internal DateTimeOffset DateFromString(string dateTimeString, XmlReader reader)
+        {
+            try
+            {
+                return DateTimeParser(dateTimeString, reader.LocalName, reader.NamespaceURI);
+            }
+            catch (FormatException e)
+            {
+                throw DiagnosticUtility.ExceptionUtility.ThrowHelperError(
+                    new XmlException(FeedUtils.AddLineInfo(reader, SR.ErrorParsingDateTime), e));
+            }
+        }
+
+        private static Uri DefaultUriParser(string value, UriKind kind, string localName, string ns)
+        {
+            return new Uri(value, kind);
+        }
+
         internal static void CloseBuffer(XmlBuffer buffer, XmlDictionaryWriter extWriter)
         {
             if (buffer == null)
diff --git a/src/System.ServiceModel.Syndication/src/System/ServiceModel/Syndication/SyndicationItem.cs b/src/System.ServiceModel.Syndication/src/System/ServiceModel/Syndication/SyndicationItem.cs
index 3f8648be02..08cf46b42e 100644
--- a/src/System.ServiceModel.Syndication/src/System/ServiceModel/Syndication/SyndicationItem.cs
+++ b/src/System.ServiceModel.Syndication/src/System/ServiceModel/Syndication/SyndicationItem.cs
@@ -158,10 +158,24 @@ namespace System.ServiceModel.Syndication
             set { _id = value; }
         }
 
+        internal Exception LastUpdatedTimeException { get; set; }
+
         public DateTimeOffset LastUpdatedTime
         {
-            get { return _lastUpdatedTime; }
-            set { _lastUpdatedTime = value; }
+            get
+            {
+                if (LastUpdatedTimeException != null)
+                {
+                    throw LastUpdatedTimeException;
+                }
+
+                return _lastUpdatedTime;
+            }
+            set
+            {
+                LastUpdatedTimeException = null;
+                _lastUpdatedTime = value;
+            }
         }
 
         public Collection<SyndicationLink> Links
@@ -176,10 +190,24 @@ namespace System.ServiceModel.Syndication
             }
         }
 
+        internal Exception PublishDateException { get; set; }
+
         public DateTimeOffset PublishDate
         {
-            get { return _publishDate; }
-            set { _publishDate = value; }
+            get
+            {
+                if (PublishDateException != null)
+                {
+                    throw PublishDateException;
+                }
+
+                return _publishDate;
+            }
+            set
+            {
+                PublishDateException = null;
+                _publishDate = value;
+            }
         }
 
         public SyndicationFeed SourceFeed
diff --git a/src/System.ServiceModel.Syndication/tests/BasicScenarioTests.cs b/src/System.ServiceModel.Syndication/tests/BasicScenarioTests.cs
index ef2665785e..ee89b27219 100644
--- a/src/System.ServiceModel.Syndication/tests/BasicScenarioTests.cs
+++ b/src/System.ServiceModel.Syndication/tests/BasicScenarioTests.cs
@@ -9,6 +9,7 @@ using System.ServiceModel.Syndication;
 using System.Xml;
 using System.IO;
 using Xunit;
+using System.Linq;
 
 namespace System.ServiceModel.Syndication.Tests
 {
@@ -285,19 +286,126 @@ namespace System.ServiceModel.Syndication.Tests
         }
 
         [Fact]
-        [ActiveIssue(25156)]
         public static void SyndicationFeed_Rss_WrongDateFormat()
         {
             // *** SETUP *** \\
-            Rss20FeedFormatter rssformatter = new Rss20FeedFormatter();
-
             XmlReader reader = XmlReader.Create(@"rssSpecExampleWrongDateFormat.xml");
 
             // *** EXECUTE *** \\
             SyndicationFeed res = SyndicationFeed.Load(reader);
 
             // *** ASSERT *** \\
-            Assert.True(!res.LastUpdatedTime.Equals(new DateTimeOffset()));
+            Assert.True(res != null, "res was null.");
+            Assert.Equal(new DateTimeOffset(2016, 8, 23, 16, 8, 0, new TimeSpan(-4, 0, 0)), res.LastUpdatedTime);
+            Assert.True(res.Items != null, "res.Items was null.");
+            Assert.True(res.Items.Count() == 4, $"res.Items.Count() was not as expected. Expected: 4; Actual: {res.Items.Count()}");
+            SyndicationItem[] items = res.Items.ToArray();
+            DateTimeOffset dateTimeOffset;
+            Assert.Throws<XmlException>(() => dateTimeOffset = items[2].PublishDate);
+        }
+
+        [Fact]
+        public static void SyndicationFeed_Rss_DateTimeParser()
+        {
+            // *** SETUP *** \\
+            // *** EXECUTE *** \\
+            SyndicationFeed feed;
+            DateTimeOffset dto = new DateTimeOffset(2017, 1, 2, 3, 4, 5, new TimeSpan(0));
+            using (XmlReader reader = XmlReader.Create(@"RssSpecCustomParser.xml"))
+            {
+                var formatter = new Rss20FeedFormatter();
+                formatter.DateTimeParser = (value, localName, ns) => dto;
+                formatter.ReadFrom(reader);
+                feed = formatter.Feed;
+            }
+
+            // *** ASSERT *** \\
+            Assert.True(feed != null, "res was null.");
+            Assert.Equal(dto, feed.LastUpdatedTime);
+        }
+
+        [Fact]
+        public static void SyndicationFeed_Rss_UriParser()
+        {
+            // *** SETUP *** \\
+            // *** EXECUTE *** \\
+            SyndicationFeed feed;
+            using (XmlReader reader = XmlReader.Create(@"RssSpecCustomParser.xml"))
+            {
+                var formatter = new Rss20FeedFormatter
+                {
+                    UriParser = (value, kind, localName, ns) => new Uri($"http://value-{value}-kind-{kind}-localName-{localName}-ns-{ns}-end")
+                };
+                formatter.ReadFrom(reader);
+                feed = formatter.Feed;
+            }
+
+            // *** ASSERT *** \\
+            Assert.True(feed != null, "res was null.");
+            Assert.Equal(new Uri("http://value-ChannelBase-kind-relativeorabsolute-localName-channel-ns--end"), feed.BaseUri);
+            Assert.Equal(new Uri("http://value-ImageUrl-kind-relativeorabsolute-localName-url-ns--end"), feed.ImageUrl);
+            Assert.NotNull(feed.Links);
+            Assert.Equal(1, feed.Links.Count);
+            Assert.Equal(new Uri("http://value-FeedLink-kind-relativeorabsolute-localName-link-ns--end"), feed.Links.First().Uri);
+
+            Assert.True(feed.Items != null, "res.Items was null.");
+            Assert.Equal(1, feed.Items.Count());
+            Assert.Equal(1, feed.Items.First().Links.Count);
+            Assert.Equal(new Uri("http://value-itemlink-kind-relativeorabsolute-localName-link-ns--end"), feed.Items.First().Links.First().Uri);
+        }
+
+        [Fact]
+        public static void SyndicationFeed_Atom_DateTimeParser()
+        {
+            // *** SETUP *** \\
+            // *** EXECUTE *** \\
+            SyndicationFeed feed;
+            DateTimeOffset dto = new DateTimeOffset(2017, 1, 2, 3, 4, 5, new TimeSpan(0));
+            using (XmlReader reader = XmlReader.Create(@"SimpleAtomFeedCustomParser.xml"))
+            {
+                var formatter = new Atom10FeedFormatter
+                {
+                    DateTimeParser = (value, localName, ns) => dto
+                };
+                formatter.ReadFrom(reader);
+                feed = formatter.Feed;
+            }
+
+            // *** ASSERT *** \\
+            Assert.True(feed != null, "res was null.");
+            Assert.Equal(dto, feed.LastUpdatedTime);
+
+            Assert.True(feed.Items != null, "res.Items was null.");
+            Assert.Equal(1, feed.Items.Count());
+            Assert.Equal(dto, feed.Items.First().LastUpdatedTime);
+        }
+
+        [Fact]
+        public static void SyndicationFeed_Atom_UriParser()
+        {
+            // *** SETUP *** \\
+            // *** EXECUTE *** \\
+            SyndicationFeed feed;
+            using (XmlReader reader = XmlReader.Create(@"SimpleAtomFeedCustomParser.xml"))
+            {
+                var formatter = new Atom10FeedFormatter
+                {
+                    UriParser = (value, kind, localName, ns) => new Uri($"http://value-{value}-kind-{kind}-localName-{localName}-ns-{ns}-end")
+                };
+                formatter.ReadFrom(reader);
+                feed = formatter.Feed;
+            }
+
+            // *** ASSERT *** \\
+            Assert.True(feed != null, "res was null.");
+            Assert.Equal(new Uri("http://value-FeedLogo-kind-relativeorabsolute-localName-logo-ns-http//www.w3.org/2005/Atom-end"), feed.ImageUrl);
+
+            Assert.True(feed.Items != null, "res.Items was null.");
+            Assert.Equal(1, feed.Items.Count());
+            Assert.NotNull(feed.Items.First().Links);
+            Assert.Equal(1, feed.Items.First().Links.Count);
+            Assert.Equal(new Uri("http://value-EntryLinkHref-kind-relativeorabsolute-localName-link-ns-http//www.w3.org/2005/Atom-end"), feed.Items.First().Links.First().Uri);
+            Assert.Equal(new Uri("http://value-EntryContentSrc-kind-relativeorabsolute-localName-content-ns-http://www.w3.org/2005/Atom-end"), ((UrlSyndicationContent)feed.Items.First().Content).Url);
         }
 
         [Fact]
diff --git a/src/System.ServiceModel.Syndication/tests/TestFeeds/RssSpecCustomParser.xml b/src/System.ServiceModel.Syndication/tests/TestFeeds/RssSpecCustomParser.xml
new file mode 100644
index 0000000000..c90436a4f7
--- /dev/null
+++ b/src/System.ServiceModel.Syndication/tests/TestFeeds/RssSpecCustomParser.xml
@@ -0,0 +1,26 @@
+<?xml version="1.0"?>
+<rss version="2.0">
+  <channel xml:base="ChannelBase" customAttribute="customAtt">
+    <title>FeedTitle</title>
+    <link>FeedLink</link>
+    <description>FeedDescription</description>
+    <language>FeedLanguage</language>
+    <lastBuildDate>Tue, 23 August 2016 16:08:00 EDT</lastBuildDate>
+    <docs>http://blogs.law.harvard.edu/tech/rss</docs>
+    <generator>FeedGenerator</generator>
+    <managingEditor>FeedEditorEmail</managingEditor>
+    <category>FeedCategory</category>
+    <copyright>FeedCopyright</copyright>
+    <ttl>60</ttl>
+    <image>
+      <url>ImageUrl</url>
+    </image>
+    <item>
+      <title>ItemTitle</title>
+      <link>ItemLink</link>
+      <description>ItemDescription</description>
+      <pubDate>Tue, 03 Jun 2003 09:39:21 GMT</pubDate>
+      <guid>ItemGuid</guid>
+    </item>
+  </channel>
+</rss>
\ No newline at end of file
diff --git a/src/System.ServiceModel.Syndication/tests/TestFeeds/SimpleAtomFeedCustomParser.xml b/src/System.ServiceModel.Syndication/tests/TestFeeds/SimpleAtomFeedCustomParser.xml
new file mode 100644
index 0000000000..e6237940dc
--- /dev/null
+++ b/src/System.ServiceModel.Syndication/tests/TestFeeds/SimpleAtomFeedCustomParser.xml
@@ -0,0 +1,23 @@
+<?xml version="1.0" encoding="utf-8"?>
+<feed xml:base="http://mypage.com/" xmlns="http://www.w3.org/2005/Atom">
+  <title type="text">FeedTitle</title>
+  <subtitle type="text">FeedSubtitle</subtitle>
+  <id>FeedID</id>
+  <updated>2017-06-26T14:41:43-07:00</updated>
+  <logo>FeedLogo</logo>
+  <generator>FeedGenerator</generator>
+  <author>
+    <name>AuthorName</name>
+    <email>author@Contoso.com</email>
+    <uri>AuthorUri</uri>
+  </author>
+  <link rel="alternate" href="http://www.contoso.com/news" />
+  <CustomElement xmlns="">asd</CustomElement>
+  <entry>
+    <id>EntryId</id>
+    <title type="text">SyndicationFeed released for .net Core</title>
+    <updated>2017-06-26T21:41:43Z</updated>
+    <link rel="alternate" href="EntryLinkHref" />
+    <content src="EntryContentSrc" />
+  </entry>
+</feed>
\ No newline at end of file