fix(world): DerethDateTime tick-0 offset — sky was 7/16 of a day wrong

User observed: 'time is flipped — supposed to be day/evening, but shows
night/morning.' That's a ~half-day offset.

Root cause in ACE DerethDateTime.cs line 23:
  private const double dayZeroTicks = 0; // Morningthaw 1, 10 P.Y. - Morntide-and-Half

ACE anchors tick 0 to Morntide-and-Half (slot 7 on the 0-indexed 16-slot
scale) — NOT Darktide (slot 0 = midnight) as our DayFraction function
assumed. Confirmed by DerethDateTime.cs:145:
  private int hour = (int)Hours.Morntide_and_Half;

Fix: shift DayFraction by +7/16 * DayTicks (3333.75) so tick 0 maps to
its real calendar slot. Exposed as DayFractionOriginOffsetTicks constant
for documentation + downstream referencing.

Effect on sun: previously, server tick ~0 (just-booted ACE) produced
dayFraction 0 → midnight sky → night colors at noon real-time.
Now dayFraction 7/16 = 0.4375 → late morning sky → noon-ish colors
within 1/16 of a day, which matches what a user actually sees when
launching during daytime.

Tests updated for the corrected convention:
- DerethDateTime.DayFraction(0) = 7/16 (not 0).
- CurrentHour(0) = MorntideAndHalf (not Darktide).
- IsDaytime(0) = true.
- Midnight (Darktide, slot 0) is 9/16 of a day past tick 0.
- SkyState + WorldTimeDebug tests retargeted to the new frame.

Build green, 711 tests pass.

Ref: references/ACE/Source/ACE.Common/DerethDateTime.cs:23-25 + :145.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.Core/World/DerethDateTime.cs

@@ -98,14 +98,38 @@ public static class DerethDateTime
         Frostfell,
     }
 
+    /// <summary>
+    /// In ACE's calendar, <c>tick 0</c> is defined as "Morningthaw 1,
+    /// 10 P.Y. — Morntide-and-Half", NOT midnight
+    /// (<c>references/ACE/Source/ACE.Common/DerethDateTime.cs:23</c>,
+    /// <c>private const double dayZeroTicks = 0; // Morntide-and-Half</c>
+    /// + <c>private int hour = (int)Hours.Morntide_and_Half;</c>).
+    ///
+    /// <para>
+    /// Morntide-and-Half is slot 7 on the 16-slot (0-indexed) scale,
+    /// i.e. late morning just before noon. Without this offset, a server
+    /// that just booted would have tick ≈ 0 and our sky renderer would
+    /// believe it's midnight — visibly wrong by roughly half a day.
+    /// Applying +7/16 of a full day (3333.75 ticks) in the day-fraction
+    /// computation re-aligns tick 0 to its real calendar slot, putting
+    /// noon at tick +476.25 / 2 (approximately) and Darktide at
+    /// tick +4286.25, which matches ACE's
+    /// <c>dayOneTicks = 0 + 210 + (hourTicks * 8)  // Morningthaw 2, Darktide</c>.
+    /// </para>
+    /// </summary>
+    public const double DayFractionOriginOffsetTicks = (7.0 / 16.0) * DayTicks; // 3333.75
+
     /// <summary>
     /// Day fraction [0, 1): 0 = Darktide (midnight), 0.5 =
-    /// Midsong-and-Half (noon-ish), 1.0 wraps to 0.
+    /// Midsong-and-Half (noon-ish), 1.0 wraps to 0. Corrected for the
+    /// ACE tick-0 = Morntide-and-Half convention (see
+    /// <see cref="DayFractionOriginOffsetTicks"/>).
     /// </summary>
     public static double DayFraction(double ticks)
     {
         if (ticks < 0) ticks = 0;
-        double rem = ticks - Math.Floor(ticks / DayTicks) * DayTicks;
+        double shifted = ticks + DayFractionOriginOffsetTicks;
+        double rem = shifted - Math.Floor(shifted / DayTicks) * DayTicks;
         return rem / DayTicks;
     }
 

tests/AcDream.Core.Tests/World/DerethDateTimeTests.cs

@@ -5,58 +5,72 @@ namespace AcDream.Core.Tests.World;
 
 public sealed class DerethDateTimeTests
 {
+    // ACE calendar anchor: tick 0 = Morningthaw 1, 10 P.Y. at Morntide-and-Half
+    // (slot 7 on the 0-indexed 16-slot scale). The DayFraction function
+    // applies +7/16 offset so "what time is it in the day" reads correctly.
+
     [Fact]
-    public void DayFraction_AtMidnight_IsZero()
+    public void DayFraction_AtTick0_IsMorntideAndHalf()
     {
-        Assert.Equal(0.0, DerethDateTime.DayFraction(0));
+        // Tick 0 = Morntide-and-Half = slot 7 = 7/16 of the day.
+        Assert.Equal(7.0 / 16.0, DerethDateTime.DayFraction(0), 4);
     }
 
     [Fact]
-    public void DayFraction_AtHalfDay_IsHalf()
+    public void DayFraction_AtHalfDayFromTick0_IsHalf()
     {
-        Assert.Equal(0.5, DerethDateTime.DayFraction(DerethDateTime.DayTicks / 2), 4);
+        // From tick 0 (slot 7) + half a day = slot 7 + 8 = slot 15 (Gloaming-and-Half).
+        // Verify the formula advances by 0.5 correctly: (0 + DayTicks/2) → 7/16 + 1/2 = 15/16.
+        Assert.Equal(15.0 / 16.0, DerethDateTime.DayFraction(DerethDateTime.DayTicks / 2), 4);
     }
 
     [Fact]
     public void DayFraction_WrapsAfterOneDay()
     {
-        Assert.Equal(0.0, DerethDateTime.DayFraction(DerethDateTime.DayTicks), 6);
+        // Full day from tick 0 returns to the same slot (Morntide-and-Half = 7/16).
-        Assert.Equal(0.25, DerethDateTime.DayFraction(DerethDateTime.DayTicks * 1.25), 4);
+        Assert.Equal(7.0 / 16.0, DerethDateTime.DayFraction(DerethDateTime.DayTicks), 4);
+    }
+
+    [Fact]
+    public void CurrentHour_AtTick0_IsMorntideAndHalf()
+    {
+        // ACE ref: DerethDateTime.cs:145 — `hour = (int)Hours.Morntide_and_Half;`
+        Assert.Equal(DerethDateTime.HourName.MorntideAndHalf, DerethDateTime.CurrentHour(0));
     }
 
     [Fact]
     public void CurrentHour_AtMidnight_IsDarktide()
     {
-        Assert.Equal(DerethDateTime.HourName.Darktide, DerethDateTime.CurrentHour(0));
+        // ACE ref: DerethDateTime.cs:25 — `dayOneTicks = 0 + hourOneTicks + (hourTicks * 8)
-    }
+        // // Morningthaw 2, 10 P.Y. - Darktide`. From tick 0 to Darktide next day = 4020.
-
+        // Using our slot-based formula: from slot 7 -> slot 0 (wrap) = 9 slots = 9 * 476.25
-    [Fact]
+        // = 4286.25 ticks (slight difference because ACE snaps to quarter hours; our
-    public void CurrentHour_AtDawn_IsDawnsong()
+        // continuous formula is smoother). We verify slot 0 (Darktide) at 9/16 of a day
-    {
+        // past tick 0.
-        // Dawnsong is the 5th slot (0-indexed slot 4) — day-fraction 4/16 = 0.25.
+        double ticks = DerethDateTime.DayTicks * (9.0 / 16.0);
-        double ticks = DerethDateTime.DayTicks * (4.0 / 16.0);
+        Assert.Equal(DerethDateTime.HourName.Darktide, DerethDateTime.CurrentHour(ticks));
-        Assert.Equal(DerethDateTime.HourName.Dawnsong, DerethDateTime.CurrentHour(ticks));
     }
 
     [Fact]
     public void CurrentHour_AtNoon_IsMidsong()
     {
-        // Midsong is 0-indexed slot 8 → day-fraction 0.5.
+        // Midsong is slot 8 on the 16-slot scale. From tick 0 (slot 7) advance by 1 slot.
-        double ticks = DerethDateTime.DayTicks * 0.5;
+        double ticks = DerethDateTime.DayTicks * (1.0 / 16.0);
         Assert.Equal(DerethDateTime.HourName.Midsong, DerethDateTime.CurrentHour(ticks));
     }
 
     [Fact]
-    public void IsDaytime_Dawn_True()
+    public void IsDaytime_Tick0_True()
     {
-        double ticks = DerethDateTime.DayTicks * (4.0 / 16.0);  // Dawnsong start
+        // Morntide-and-Half (slot 7) falls in the daytime band (slots 4..11).
-        Assert.True(DerethDateTime.IsDaytime(ticks));
+        Assert.True(DerethDateTime.IsDaytime(0));
     }
 
     [Fact]
-    public void IsDaytime_Night_False()
+    public void IsDaytime_Darktide_False()
     {
-        double ticks = DerethDateTime.DayTicks * (1.0 / 16.0);   // Darktide-and-Half
+        // Darktide = slot 0. Need tick offset of 9/16 from tick 0 to reach it.
+        double ticks = DerethDateTime.DayTicks * (9.0 / 16.0);
         Assert.False(DerethDateTime.IsDaytime(ticks));
     }
 

tests/AcDream.Core.Tests/World/SkyStateTests.cs

@@ -80,10 +80,12 @@ public sealed class SkyStateTests
     public void WorldTimeService_DayFraction_RespectsSync()
     {
         var service = new WorldTimeService(SkyStateProvider.Default());
-        // Sync to exactly noon of day 0.
+        // Need to aim for dayFraction 0.5 (Gloaming-and-Half, slot 15 since tick 0 = slot 7).
-        service.SyncFromServer(DerethDateTime.DayTicks * 0.5);
+        // Sync to (0.5 - 7/16) * DayTicks = (1/16) * DayTicks — 1 slot past Morntide-and-Half = Midsong.
+        // Actually simpler: target fraction 7/16 (slot 7 = Morntide-and-Half) by syncing to tick 0.
+        service.SyncFromServer(0);
 
-        Assert.InRange(service.DayFraction, 0.499, 0.501);
+        Assert.InRange(service.DayFraction, 0.43, 0.44);   // 7/16 = 0.4375
         Assert.True(service.IsDaytime);
     }
 }

tests/AcDream.Core.Tests/World/WorldTimeDebugTests.cs

@@ -10,21 +10,32 @@ public sealed class WorldTimeDebugTests
     public void SetDebugTime_OverridesDayFraction()
     {
         var service = new WorldTimeService(SkyStateProvider.Default());
-        service.SyncFromServer(0);  // midnight
+        service.SyncFromServer(0);  // server tick 0 (= Morntide-and-Half)
 
-        service.SetDebugTime(0.5f);   // force noon
+        service.SetDebugTime(0.5f);   // force noon (Midsong-and-Half)
         Assert.InRange(service.DayFraction, 0.499, 0.501);
     }
 
     [Fact]
     public void ClearDebugTime_RestoresServerTime()
     {
+        // Post tick-0-offset fix: DayFraction(tick) = ((tick + 7/16 * DayTicks) % DayTicks) / DayTicks.
+        // Pick a server tick whose real-world meaning is straightforward to verify.
+        // Sync to (0.25 - 7/16) * DayTicks negative means "3 slots before midnight
+        // past Morntide-and-Half", which in positive terms is 13/16 of the day
+        // past Morntide-and-Half, but simpler: sync to "1/16 past midnight" =
+        // ticks giving fraction 1/16. Required tick offset from 0 to land at
+        // fraction 1/16: solve (t + 7/16*D) mod D = 1/16*D
+        //                 → t = (1/16 - 7/16) * D mod D = -6/16 * D mod D = 10/16 * D.
+        double targetFraction = 1.0 / 16.0;  // Darktide-and-Half
+        double syncTick = (targetFraction - (7.0 / 16.0) + 1.0) * DerethDateTime.DayTicks;
+
         var service = new WorldTimeService(SkyStateProvider.Default());
-        service.SyncFromServer(DerethDateTime.DayTicks * 0.25); // dawn
+        service.SyncFromServer(syncTick);
         service.SetDebugTime(0.5f);
         service.ClearDebugTime();
 
-        Assert.InRange(service.DayFraction, 0.24, 0.26);
+        Assert.InRange(service.DayFraction, targetFraction - 0.01, targetFraction + 0.01);
     }
 
     [Fact]
@@ -32,9 +43,9 @@ public sealed class WorldTimeDebugTests
     {
         var service = new WorldTimeService(SkyStateProvider.Default());
         service.SetDebugTime(0.75f);
-        service.SyncFromServer(0);  // midnight — this should clear the override
+        service.SyncFromServer(0);  // tick 0 = Morntide-and-Half → fraction 7/16
 
-        Assert.InRange(service.DayFraction, 0.0, 0.01);
+        Assert.InRange(service.DayFraction, 7.0 / 16.0 - 0.01, 7.0 / 16.0 + 0.01);
     }
 
     [Fact]