Expose Adjustment Rule BaseUtcOffsetDelta

src/libraries/System.Private.CoreLib/src/System/TimeZoneInfo.AdjustmentRule.cs

@@ -31,7 +31,10 @@ public sealed class AdjustmentRule : IEquatable<AdjustmentRule?>, ISerializable,
 
             public TransitionTime DaylightTransitionEnd => _daylightTransitionEnd;
 
-            internal TimeSpan BaseUtcOffsetDelta => _baseUtcOffsetDelta;
+            /// <summary>
+            /// Gets the time difference with the particular time zone base Utc offset during the adjustment rule period.
+            /// </summary>
+            public TimeSpan BaseUtcOffsetDelta => _baseUtcOffsetDelta;
 
             /// <summary>
             /// Gets a value indicating that this AdjustmentRule fixes the time zone offset
@@ -76,7 +79,17 @@ private AdjustmentRule(
                 _noDaylightTransitions = noDaylightTransitions;
             }
 
-            internal static AdjustmentRule CreateAdjustmentRule(
+            /// <summary>
+            /// Creates a new adjustment rule for a particular time zone.
+            /// </summary>
+            /// <param name="dateStart">The effective date of the adjustment rule. If the value of the dateStart parameter is DateTime.MinValue.Date, this is the first adjustment rule in effect for a time zone.</param>
+            /// <param name="dateEnd">The last date that the adjustment rule is in force. If the value of the dateEnd parameter is DateTime.MaxValue.Date, the adjustment rule has no end date.</param>
+            /// <param name="daylightDelta">The time change that results from the adjustment. This value is added to the time zone's BaseUtcOffset+BaseUtcOffsetDelta properties to obtain the correct daylight offset from Coordinated Universal Time (UTC). This value can range from -14 to 14.</param>
+            /// <param name="daylightTransitionStart">An object that defines the start of daylight saving time.</param>
+            /// <param name="daylightTransitionEnd">An object that defines the end of daylight saving time.</param>
+            /// <param name="baseUtcOffsetDelta">The time difference with the particular time zone base Utc offset during this adjustment rule period.</param>
+            /// <returns>An object that represents the new adjustment rule.</returns>
+            public static AdjustmentRule CreateAdjustmentRule(
                 DateTime dateStart,
                 DateTime dateEnd,
                 TimeSpan daylightDelta,

src/libraries/System.Runtime/ref/System.Runtime.cs

@@ -3896,9 +3896,11 @@ internal AdjustmentRule() { }
             public System.DateTime DateEnd { get { throw null; } }
             public System.DateTime DateStart { get { throw null; } }
             public System.TimeSpan DaylightDelta { get { throw null; } }
+            public System.TimeSpan BaseUtcOffsetDelta { get { throw null; } }
             public System.TimeZoneInfo.TransitionTime DaylightTransitionEnd { get { throw null; } }
             public System.TimeZoneInfo.TransitionTime DaylightTransitionStart { get { throw null; } }
             public static System.TimeZoneInfo.AdjustmentRule CreateAdjustmentRule(System.DateTime dateStart, System.DateTime dateEnd, System.TimeSpan daylightDelta, System.TimeZoneInfo.TransitionTime daylightTransitionStart, System.TimeZoneInfo.TransitionTime daylightTransitionEnd) { throw null; }
+            public static System.TimeZoneInfo.AdjustmentRule CreateAdjustmentRule(System.DateTime dateStart, System.DateTime dateEnd, System.TimeSpan daylightDelta, System.TimeZoneInfo.TransitionTime daylightTransitionStart, System.TimeZoneInfo.TransitionTime daylightTransitionEnd, System.TimeSpan baseUtcOffsetDelta) { throw null; }
             public bool Equals([System.Diagnostics.CodeAnalysis.NotNullWhenAttribute(true)] System.TimeZoneInfo.AdjustmentRule? other) { throw null; }
             public override int GetHashCode() { throw null; }
             void System.Runtime.Serialization.IDeserializationCallback.OnDeserialization(object? sender) { }

src/libraries/System.Runtime/tests/System/TimeZoneInfoTests.cs

@@ -2314,7 +2314,7 @@ public static IEnumerable<object[]> SystemTimeZonesTestData()
                 for (int i = -14; i <= 12; i++)
                 {
                     TimeZoneInfo tz = null;
- 
+
                     try
                     {
                         string id = $"Etc/GMT{i:+0;-0}";
@@ -2706,6 +2706,32 @@ public static void ChangeLocalTimeZone(string id)
             }
         }
 
+        [Fact]
+        public static void AdjustmentRuleBaseUtcOffsetDeltaTest()
+        {
+            TimeZoneInfo.TransitionTime start = TimeZoneInfo.TransitionTime.CreateFixedDateRule(timeOfDay: new DateTime(1, 1, 1, 2, 0, 0), month: 3, day: 7);
+            TimeZoneInfo.TransitionTime end = TimeZoneInfo.TransitionTime.CreateFixedDateRule(timeOfDay: new DateTime(1, 1, 1, 1, 0, 0), month: 11, day: 7);
+            TimeZoneInfo.AdjustmentRule rule = TimeZoneInfo.AdjustmentRule.CreateAdjustmentRule(DateTime.MinValue.Date, DateTime.MaxValue.Date, new TimeSpan(1, 0, 0), start, end, baseUtcOffsetDelta: new TimeSpan(1, 0, 0));
+            TimeZoneInfo customTimeZone = TimeZoneInfo.CreateCustomTimeZone(
+                                                            id: "Fake Time Zone",
+                                                            baseUtcOffset: new TimeSpan(0),
+                                                            displayName: "Fake Time Zone",
+                                                            standardDisplayName: "Standard Fake Time Zone",
+                                                            daylightDisplayName: "British Summer Time",
+                                                            new TimeZoneInfo.AdjustmentRule[] { rule });
+
+            TimeZoneInfo.AdjustmentRule[] rules = customTimeZone.GetAdjustmentRules();
+
+            Assert.Equal(1, rules.Length);
+            Assert.Equal(new TimeSpan(1, 0, 0), rules[0].BaseUtcOffsetDelta);
+
+            // BaseUtcOffsetDelta should be counted to the returned offset during the standard time.
+            Assert.Equal(new TimeSpan(1, 0, 0), customTimeZone.GetUtcOffset(new DateTime(2021, 1, 1, 2, 0, 0)));
+
+            // BaseUtcOffsetDelta should be counted to the returned offset during the daylight time.
+            Assert.Equal(new TimeSpan(2, 0, 0), customTimeZone.GetUtcOffset(new DateTime(2021, 3, 10, 2, 0, 0)));
+        }
+
         private static bool IsEnglishUILanguage => CultureInfo.CurrentUICulture.Name.Length == 0 || CultureInfo.CurrentUICulture.TwoLetterISOLanguageName == "en";
 
         private static bool IsEnglishUILanguageAndRemoteExecutorSupported => IsEnglishUILanguage && RemoteExecutor.IsSupported;