fix block start time calculation

CHANGELOG.md

@@ -21,6 +21,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 - Fix playback failure when seeking content with certain DTS audio (e.g. DTS-HD MA)
 - Properly set explicit audio decoder on combined audio and video input file
 - Fix building sequential schedules across a UTC offset change
+- Fix block start time calculation across a UTC offset change
 
 ## [25.8.0] - 2025-10-26
 ### Added

ErsatzTV.Core.Tests/Scheduling/BlockScheduling/EffectiveBlockTests.cs

@@ -101,9 +101,82 @@ public static class EffectiveBlockTests
             result[2].Start.Date.ShouldBe(GetLocalDate(2024, 1, 19).Date);
         }
 
-        // TODO: test when clocks spring forward
-        // TODO: test when clocks fall back
+        [Test]
+        public void Should_Handle_Spring_Forward()
+        {
+            DateTimeOffset now = DateTimeOffset.Now;
+
+            List<PlayoutTemplate> templates =
+            [
+                new()
+                {
+                    Index = 1,
+                    DaysOfWeek = PlayoutTemplate.AllDaysOfWeek(),
+                    DaysOfMonth = PlayoutTemplate.AllDaysOfMonth(),
+                    MonthsOfYear = PlayoutTemplate.AllMonthsOfYear(),
+                    Template = SingleBlockTemplate(now), // 9am block
+                    DateUpdated = now.UtcDateTime
+                }
+            ];
+
+            // In 2024, DST starts on March 10 for America/Chicago
+            // For Windows, this would be "Central Standard Time"
+            var tz = TimeZoneInfo.FindSystemTimeZoneById("America/Chicago");
+            var start = new DateTime(2024, 3, 9, 0, 0, 0, DateTimeKind.Unspecified);
+            var dto = new DateTimeOffset(start, tz.GetUtcOffset(start));
+
+            List<EffectiveBlock> result = EffectiveBlock.GetEffectiveBlocks(templates, dto, 5);
+
+            result.Count.ShouldBe(5);
+
+            // Saturday March 9, 9am is CST (-6)
+            var blockOnSat = result.Single(r => r.Start.Day == 9);
+            blockOnSat.Start.Hour.ShouldBe(9);
+            blockOnSat.Start.Offset.ShouldBe(TimeSpan.FromHours(-6));
+
+            // Sunday March 10, 9am is CDT (-5)
+            var blockOnSun = result.Single(r => r.Start.Day == 10);
+            blockOnSun.Start.Hour.ShouldBe(9);
+            blockOnSun.Start.Offset.ShouldBe(TimeSpan.FromHours(-5));
+        }
 
-        // TODO: offset may be incorrect on days with time change, since start offset is re-used
+        [Test]
+        public void Should_Handle_Fall_Back()
+        {
+            DateTimeOffset now = DateTimeOffset.Now;
+
+            List<PlayoutTemplate> templates =
+            [
+                new()
+                {
+                    Index = 1,
+                    DaysOfWeek = PlayoutTemplate.AllDaysOfWeek(),
+                    DaysOfMonth = PlayoutTemplate.AllDaysOfMonth(),
+                    MonthsOfYear = PlayoutTemplate.AllMonthsOfYear(),
+                    Template = SingleBlockTemplate(now), // 9am block
+                    DateUpdated = now.UtcDateTime
+                }
+            ];
+
+            // In 2024, DST ends on Nov 3 for America/Chicago
+            // For Windows, this would be "Central Standard Time"
+            var tz = TimeZoneInfo.FindSystemTimeZoneById("America/Chicago");
+            var start = new DateTime(2024, 11, 2, 0, 0, 0, DateTimeKind.Unspecified);
+            var dto = new DateTimeOffset(start, tz.GetUtcOffset(start));
+
+            List<EffectiveBlock> result = EffectiveBlock.GetEffectiveBlocks(templates, dto, 5);
+
+            result.Count.ShouldBe(5);
+
+            // Saturday Nov 2, 9am is CDT (-5)
+            var blockOnSat = result.Single(r => r.Start.Day == 2);
+            blockOnSat.Start.Hour.ShouldBe(9);
+            blockOnSat.Start.Offset.ShouldBe(TimeSpan.FromHours(-5));
+
+            // Sunday Nov 3, 9am is CST (-6)
+            var blockOnSun = result.Single(r => r.Start.Day == 3);
+            blockOnSun.Start.Hour.ShouldBe(9);
+            blockOnSun.Start.Offset.ShouldBe(TimeSpan.FromHours(-6));
+        }
     }
 }

ErsatzTV.Core.Tests/Scheduling/ClassicScheduling/PlayoutBuilderTestBase.cs

@@ -37,14 +37,15 @@ public abstract class PlayoutBuilderTestBase
 
     protected static DateTimeOffset HoursAfterMidnight(int hours)
     {
-        DateTimeOffset now = DateTimeOffset.Now;
-        return now - now.TimeOfDay + TimeSpan.FromHours(hours);
-
-        // // pick a timezone that has DST and a known offset on a specific date
-        // TimeZoneInfo eastern = TimeZoneInfo.FindSystemTimeZoneById("America/New_York");
-        // DateTime date = new DateTime(2025, 11, 2, 0, 0, 0, DateTimeKind.Unspecified);
-        // DateTimeOffset now = new DateTimeOffset(date, eastern.GetUtcOffset(date));
-        // return now.Date + TimeSpan.FromHours(hours);
+        // DateTimeOffset now = DateTimeOffset.Now;
+        // return now - now.TimeOfDay + TimeSpan.FromHours(hours);
+
+        // pick a timezone that has DST and a known offset on a specific date
+        TimeZoneInfo eastern = TimeZoneInfo.FindSystemTimeZoneById("America/New_York");
+        //DateTime date = new DateTime(2025, 11, 2, 0, 0, 0, DateTimeKind.Unspecified);
+        DateTime date = new DateTime(2025, 10, 4, 0, 0, 0, DateTimeKind.Unspecified);
+        DateTimeOffset now = new DateTimeOffset(date, eastern.GetUtcOffset(date));
+        return now.Date + TimeSpan.FromHours(hours);
     }
 
     protected TestData TestDataFloodForItems(

ErsatzTV.Core/Scheduling/BlockScheduling/EffectiveBlock.cs

@@ -9,6 +9,8 @@ internal record EffectiveBlock(Block Block, BlockKey BlockKey, DateTimeOffset St
         DateTimeOffset start,
         int daysToBuild)
     {
+        var timeZone = TimeZoneInfo.Local;
+
         DateTimeOffset finish = start.AddDays(daysToBuild);
 
         var effectiveBlocks = new List<EffectiveBlock>();
@@ -26,14 +28,15 @@ internal record EffectiveBlock(Block Block, BlockKey BlockKey, DateTimeOffset St
                 DateTimeOffset today = current;
 
                 var newBlocks = playoutTemplate.Template.Items
-                    .Map(i => ToEffectiveBlock(playoutTemplate, i, today, start))
+                    .Map(i => ToEffectiveBlock(playoutTemplate, i, today))
                     .Map(NormalizeGuideMode)
                     .ToList();
 
                 effectiveBlocks.AddRange(newBlocks);
             }
 
-            current = current.AddDays(1);
+            var localDate = TimeZoneInfo.ConvertTime(current, timeZone).Date;
+            current = new DateTimeOffset(localDate.AddDays(1), timeZone.GetUtcOffset(localDate.AddDays(1)));
         }
 
         effectiveBlocks.RemoveAll(b => b.Start.AddMinutes(b.Block.Minutes) < start || b.Start > finish);
@@ -45,20 +48,27 @@ internal record EffectiveBlock(Block Block, BlockKey BlockKey, DateTimeOffset St
     private static EffectiveBlock ToEffectiveBlock(
         PlayoutTemplate playoutTemplate,
         TemplateItem templateItem,
-        DateTimeOffset current,
-        DateTimeOffset start) =>
-        new(
+        DateTimeOffset current)
+    {
+        var blockStartTime = new DateTime(
+            current.Year,
+            current.Month,
+            current.Day,
+            templateItem.StartTime.Hours,
+            templateItem.StartTime.Minutes,
+            0,
+            DateTimeKind.Unspecified);
+
+        // use the system's local time zone
+        var timeZone = TimeZoneInfo.Local;
+        var blockStart = new DateTimeOffset(blockStartTime, timeZone.GetUtcOffset(blockStartTime));
+
+        return new EffectiveBlock(
             templateItem.Block,
             new BlockKey(templateItem.Block, templateItem.Template, playoutTemplate),
-            new DateTimeOffset(
-                current.Year,
-                current.Month,
-                current.Day,
-                templateItem.StartTime.Hours,
-                templateItem.StartTime.Minutes,
-                0,
-                start.Offset),
+            blockStart,
             templateItem.Id);
+    }
 
     private static EffectiveBlock NormalizeGuideMode(EffectiveBlock effectiveBlock)
     {