1 files changed, 25 insertions, 2 deletions

diff --git a/src/System.Private.CoreLib/shared/System/TimeZoneInfo.AdjustmentRule.cs b/src/System.Private.CoreLib/shared/System/TimeZoneInfo.AdjustmentRule.cs
index 0e949a30e..aceb7b90f 100644
--- a/src/System.Private.CoreLib/shared/System/TimeZoneInfo.AdjustmentRule.cs
+++ b/src/System.Private.CoreLib/shared/System/TimeZoneInfo.AdjustmentRule.cs
@@ -11,6 +11,8 @@ namespace System
         [Serializable]
         public sealed class AdjustmentRule : IEquatable<AdjustmentRule>, ISerializable, IDeserializationCallback
         {
+            private static readonly TimeSpan DaylightDeltaAdjustment = TimeSpan.FromHours(24.0);
+            private static readonly TimeSpan MaxDaylightDelta = TimeSpan.FromHours(12.0);
             private readonly DateTime _dateStart;
             private readonly DateTime _dateEnd;
             private readonly TimeSpan _daylightDelta;
@@ -39,8 +41,8 @@ namespace System
 
             internal bool HasDaylightSaving =>
                 DaylightDelta != TimeSpan.Zero ||
-                (DaylightTransitionStart != default(TransitionTime) && DaylightTransitionStart.TimeOfDay != DateTime.MinValue) ||
-                (DaylightTransitionEnd != default(TransitionTime) && DaylightTransitionEnd.TimeOfDay != DateTime.MinValue.AddMilliseconds(1));
+                (DaylightTransitionStart != default && DaylightTransitionStart.TimeOfDay != DateTime.MinValue) ||
+                (DaylightTransitionEnd != default && DaylightTransitionEnd.TimeOfDay != DateTime.MinValue.AddMilliseconds(1));
 
             public bool Equals(AdjustmentRule other) =>
                 other != null &&
@@ -100,6 +102,7 @@ namespace System
                 TimeSpan baseUtcOffsetDelta,
                 bool noDaylightTransitions)
             {
+                AdjustDaylightDeltaToExpectedRange(ref daylightDelta, ref baseUtcOffsetDelta);
                 return new AdjustmentRule(
                     dateStart,
                     dateEnd,
@@ -186,6 +189,26 @@ namespace System
                 }
             }
 
+            /// <summary>
+            /// Ensures the daylight delta is within [-12, 12] hours
+            /// </summary>>
+            private static void AdjustDaylightDeltaToExpectedRange(ref TimeSpan daylightDelta, ref TimeSpan baseUtcOffsetDelta)
+            {
+                if (daylightDelta > MaxDaylightDelta)
+                {
+                    daylightDelta -= DaylightDeltaAdjustment;
+                    baseUtcOffsetDelta += DaylightDeltaAdjustment;
+                }
+                else if (daylightDelta < -MaxDaylightDelta)
+                {
+                    daylightDelta += DaylightDeltaAdjustment;
+                    baseUtcOffsetDelta -= DaylightDeltaAdjustment;
+                }
+
+                System.Diagnostics.Debug.Assert(daylightDelta <= MaxDaylightDelta && daylightDelta >= -MaxDaylightDelta,
+                                                "DaylightDelta should not ever be more than 24h");
+            }
+
             void IDeserializationCallback.OnDeserialization(object sender)
             {
                 // OnDeserialization is called after each instance of this class is deserialized.