Eliminate extra time zone conversions in DateTimeOffset (#25658)

* Eliminate extra time zone conversions in DateTimeOffset

Revised members dealing specifically with local time by removing delegation to DateTime methods to avoid an extra call to TimeZoneInfo to determine offset:
Now
FromFileTime(long)
ToLocalTime()

* Fix DateTimeOffset.ToLocalTime(bool)

* Consolidate conversion from UTC DateTime to Local DateTimeOffset

Signed-off-by: dotnet-bot <dotnet-bot@microsoft.com>

1 files changed, 18 insertions, 3 deletions

diff --git a/netcore/System.Private.CoreLib/shared/System/DateTimeOffset.cs b/netcore/System.Private.CoreLib/shared/System/DateTimeOffset.cs
index 6cc68de64d4..f3a604ca7cd 100644
--- a/netcore/System.Private.CoreLib/shared/System/DateTimeOffset.cs
+++ b/netcore/System.Private.CoreLib/shared/System/DateTimeOffset.cs
@@ -176,7 +176,7 @@ namespace System
         {
             get
             {
-                return new DateTimeOffset(DateTime.Now);
+                return ToLocalTime(DateTime.UtcNow, true);
             }
         }
 
@@ -559,7 +559,7 @@ namespace System
         //
         public static DateTimeOffset FromFileTime(long fileTime)
         {
-            return new DateTimeOffset(DateTime.FromFileTime(fileTime));
+            return ToLocalTime(DateTime.FromFileTimeUtc(fileTime), true);
         }
 
         public static DateTimeOffset FromUnixTimeSeconds(long seconds)
@@ -792,7 +792,22 @@ namespace System
 
         internal DateTimeOffset ToLocalTime(bool throwOnOverflow)
         {
-            return new DateTimeOffset(UtcDateTime.ToLocalTime(throwOnOverflow));
+            return ToLocalTime(UtcDateTime, throwOnOverflow);
+        }
+
+        private static DateTimeOffset ToLocalTime(DateTime utcDateTime, bool throwOnOverflow)
+        {
+            TimeSpan offset = TimeZoneInfo.GetLocalUtcOffset(utcDateTime, TimeZoneInfoOptions.NoThrowOnInvalidTime);
+            long localTicks = utcDateTime.Ticks + offset.Ticks;
+            if (localTicks < DateTime.MinTicks || localTicks > DateTime.MaxTicks)
+            {
+                if (throwOnOverflow)
+                    throw new ArgumentException(SR.Arg_ArgumentOutOfRangeException);
+
+                localTicks = localTicks < DateTime.MinTicks ? DateTime.MinTicks : DateTime.MaxTicks;
+            }
+            
+            return new DateTimeOffset(localTicks, offset);
         }
 
         public override string ToString()