Added unit tests to DateTest.cs exercising the edge cases where the start/end times are exactly equal to the daylight savings time transition window start/end

Updated ComputeRealRange to detect when start and end are in the same window, and to handle the edge case where start is exactly the last valid date/time before the window.

Author: logiclrd

Source/Bogus.Tests/DataSetTests/DateTest.cs

@@ -460,6 +460,90 @@ public void will_adjust_end_time_to_avoid_dst_transition()
          }
       }
 
+      [FactWhenDaylightSavingsSupported]
+      public void works_when_range_is_exactly_daylight_savings_transition_window()
+      {
+         // Arrange
+         var faker = new Faker();
+
+         faker.Random = new Randomizer(localSeed: 5);
+
+         var dstRules = TimeZoneInfo.Local.GetAdjustmentRules();
+
+         var now = DateTime.Now;
+
+         var effectiveRule = dstRules.Single(rule => (rule.DateStart <= now) && (rule.DateEnd >= now));
+
+         var transitionStartTime = CalculateTransitionDateTime(now, effectiveRule.DaylightTransitionStart);
+         var transitionEndTime = transitionStartTime + effectiveRule.DaylightDelta;
+
+         // Act & Assert
+         for (int i = 0; i < 10000; i++)
+         {
+            var sample = faker.Date.Between(transitionStartTime, transitionEndTime);
+
+            sample.Should().BeOneOf(transitionStartTime, transitionEndTime);
+         }
+      }
+
+      [FactWhenDaylightSavingsSupported]
+      public void works_when_range_start_is_exactly_daylight_savings_transition_window_start()
+      {
+         // Arrange
+         var faker = new Faker();
+
+         faker.Random = new Randomizer(localSeed: 5);
+
+         var dstRules = TimeZoneInfo.Local.GetAdjustmentRules();
+
+         var now = DateTime.Now;
+
+         var effectiveRule = dstRules.Single(rule => (rule.DateStart <= now) && (rule.DateEnd >= now));
+
+         var transitionStartTime = CalculateTransitionDateTime(now, effectiveRule.DaylightTransitionStart);
+         var transitionEndTime = transitionStartTime + effectiveRule.DaylightDelta;
+
+         var windowStart = transitionStartTime;
+         var windowEnd = transitionEndTime.AddMinutes(-5);
+
+         // Act & Assert
+         for (int i = 0; i < 10000; i++)
+         {
+            var sample = faker.Date.Between(windowStart, windowEnd);
+
+            sample.Should().Be(windowStart);
+         }
+      }
+
+      [FactWhenDaylightSavingsSupported]
+      public void works_when_range_end_is_exactly_daylight_savings_transition_window_end()
+      {
+         // Arrange
+         var faker = new Faker();
+
+         faker.Random = new Randomizer(localSeed: 5);
+
+         var dstRules = TimeZoneInfo.Local.GetAdjustmentRules();
+
+         var now = DateTime.Now;
+
+         var effectiveRule = dstRules.Single(rule => (rule.DateStart <= now) && (rule.DateEnd >= now));
+
+         var transitionStartTime = CalculateTransitionDateTime(now, effectiveRule.DaylightTransitionStart);
+         var transitionEndTime = transitionStartTime + effectiveRule.DaylightDelta;
+
+         var windowStart = transitionStartTime.AddMinutes(5);
+         var windowEnd = transitionEndTime;
+
+         // Act & Assert
+         for (int i = 0; i < 10000; i++)
+         {
+            var sample = faker.Date.Between(windowStart, windowEnd);
+
+            sample.Should().Be(windowEnd);
+         }
+      }
+
       private DateTime CalculateTransitionDateTime(DateTime now, TimeZoneInfo.TransitionTime transition)
       {
          // Based on code found at: https://docs.microsoft.com/en-us/dotnet/api/system.timezoneinfo.transitiontime.isfixeddaterule

Source/Bogus/DataSets/Date.cs

@@ -144,8 +144,17 @@ public DateTime Between(DateTime start, DateTime end)
          var value = new DateTime(minTicks, DateTimeKind.Utc) + partTimeSpan;
 
          if (start.Kind != DateTimeKind.Utc)
+         {
             value = value.ToLocalTime();
 
+            // Right around daylight savings time transition, there can be two different local DateTime values
+            // that are actually exactly the same DateTime. The ToLocalTime conversion might pick the wrong
+            // one in edge cases; it will pick the later one, and if the caller's window includes the earlier
+            // one, we should return that instead to follow the principle of least surprise.
+            if (value > end)
+               value = end;
+         }
+
          return value;
       }
 
@@ -178,12 +187,20 @@ private void ComputeRealRange(ref DateTime start, ref DateTime end)
 #if !NETSTANDARD1_3
          var window = GetForwardDSTTransitionWindow(start);
 
-         if ((start > window.Start) && (start <= window.End))
-            start = new DateTime(window.End.Ticks, start.Kind);
+         if ((start >= window.Start) && (start <= window.End))
+         {
+            if ((start == window.Start) && (end >= window.Start) && (end <= window.End))
+               end = start;
+            else
+               start = new DateTime(window.End.Ticks, start.Kind);
+
+            if (start == end)
+               return;
+         }
 
          window = GetForwardDSTTransitionWindow(end);
 
-         if ((end >= window.Start) && (end < window.End))
+         if ((end >= window.Start) && (end <= window.End))
             end = new DateTime(window.Start.Ticks, end.Kind);
 
          if (start > end)