diff --git a/src/AcDream.App/Rendering/GameWindow.cs b/src/AcDream.App/Rendering/GameWindow.cs
index d2a63aa..d8230c5 100644
--- a/src/AcDream.App/Rendering/GameWindow.cs
+++ b/src/AcDream.App/Rendering/GameWindow.cs
@@ -893,6 +893,26 @@ public sealed class GameWindow : IDisposable
                 // 2026-04-23).
                 WorldTime.TickSize = 1.0;
 
+                // Phase 3f: adopt the dat's GameTime.ZeroTimeOfYear as the
+                // calendar-extraction offset. Dereth's dat value is 3600
+                // (verified 2026-04-23 live dump); ACE's DerethDateTime.cs
+                // comment that "tick 0 = Morntide-and-Half" (3333.75
+                // offset = +7/16) is WRONG by 266.25 ticks against the
+                // authoritative dat. The mismatch cascaded into both the
+                // wrong hour label AND the wrong DayOfYear at boundary
+                // times (different LCG seed → different DayGroup roll),
+                // which explained the user's observation of "acdream
+                // clear night, retail stormy pre-dawn" at the same
+                // server PortalYearTicks.
+                if (region.GameTime is not null)
+                {
+                    AcDream.Core.World.DerethDateTime.SetOriginOffsetFromDat(
+                        region.GameTime.ZeroTimeOfYear);
+                    Console.WriteLine(
+                        $"sky: GameTime ZeroTimeOfYear={region.GameTime.ZeroTimeOfYear} " +
+                        $"(was default {AcDream.Core.World.DerethDateTime.DayFractionOriginOffsetTicks})");
+                }
+
                 Console.WriteLine(
                     $"sky: loaded Region 0x13000000 — {_loadedSkyDesc.DayGroups.Count} day groups, " +
                     $"SkyDesc.TickSize={_loadedSkyDesc.TickSize} (throttle, not rate), " +
diff --git a/src/AcDream.Core/World/DerethDateTime.cs b/src/AcDream.Core/World/DerethDateTime.cs
index 0ee9c2d..592b868 100644
--- a/src/AcDream.Core/World/DerethDateTime.cs
+++ b/src/AcDream.Core/World/DerethDateTime.cs
@@ -108,36 +108,54 @@ public static class DerethDateTime
     }
 
     /// <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>
+    /// Default anchor ticks when no Region has been loaded: +7/16 of a
+    /// full day (3333.75) per ACE's DerethDateTime comment "tick 0 =
+    /// Morntide-and-Half". This is a fallback — retail reads
+    /// <c>GameTime.ZeroTimeOfYear</c> from the Region dat (verified
+    /// <c>3600</c> in Dereth, 2026-04-23 live dump) and uses that as
+    /// the additive offset. We override <see cref="OriginOffsetTicks"/>
+    /// once the dat loads; this constant is only for offline tests.
     /// </summary>
     public const double DayFractionOriginOffsetTicks = (7.0 / 16.0) * DayTicks; // 3333.75
 
+    /// <summary>
+    /// Additive tick offset applied before every calendar extraction
+    /// (DayFraction / Year / DayOfYear / AbsoluteYear). Populated from
+    /// the Region dat's <c>GameTime.ZeroTimeOfYear</c> via
+    /// <see cref="SetOriginOffsetFromDat"/> at Region load. Defaults to
+    /// <see cref="DayFractionOriginOffsetTicks"/> for offline tests and
+    /// for the boot window before the dat parses.
+    ///
+    /// <para>
+    /// Live Dereth dat value: <c>3600</c>. The +7/16 default is wrong
+    /// by 266.25 ticks (~33 Derethian minutes) and was the source of
+    /// the "acdream time is behind retail" + "wrong DayGroup picked"
+    /// observations in the 2026-04-23 live verification session — see
+    /// <c>docs/research/2026-04-23-daygroup-selection.md</c> §4 and
+    /// the Phase 3f commit.
+    /// </para>
+    /// </summary>
+    public static double OriginOffsetTicks { get; private set; } = DayFractionOriginOffsetTicks;
+
+    /// <summary>
+    /// Adopt the Region dat's <c>GameTime.ZeroTimeOfYear</c> as the
+    /// calendar-extraction offset. Idempotent; safe to call on every
+    /// Region reload (though in practice Dereth is the only region).
+    /// </summary>
+    public static void SetOriginOffsetFromDat(double zeroTimeOfYear)
+    {
+        OriginOffsetTicks = zeroTimeOfYear;
+    }
+
     /// <summary>
     /// Day fraction [0, 1): 0 = Darktide (midnight), 0.5 =
-    /// Midsong-and-Half (noon-ish), 1.0 wraps to 0. Corrected for the
-    /// ACE tick-0 = Morntide-and-Half convention (see
-    /// <see cref="DayFractionOriginOffsetTicks"/>).
+    /// Midsong-and-Half (noon-ish), 1.0 wraps to 0. Anchored by
+    /// <see cref="OriginOffsetTicks"/> (dat's ZeroTimeOfYear).
     /// </summary>
     public static double DayFraction(double ticks)
     {
         if (ticks < 0) ticks = 0;
-        double shifted = ticks + DayFractionOriginOffsetTicks;
+        double shifted = ticks + OriginOffsetTicks;
         double rem = shifted - Math.Floor(shifted / DayTicks) * DayTicks;
         return rem / DayTicks;
     }
@@ -175,8 +193,9 @@ public static class DerethDateTime
     public static Calendar ToCalendar(double ticks)
     {
         if (ticks < 0) ticks = 0;
-        int year = (int)(ticks / YearTicks);
-        double tYear = ticks - year * YearTicks;
+        double shifted = ticks + OriginOffsetTicks;
+        int year = (int)(shifted / YearTicks);
+        double tYear = shifted - year * YearTicks;
         int monthIdx = (int)(tYear / MonthTicks);
         if (monthIdx > 11) monthIdx = 11;
         double tMonth = tYear - monthIdx * MonthTicks;
@@ -188,15 +207,17 @@ public static class DerethDateTime
 
     /// <summary>
     /// Year offset from tick-0 (number of Derethian years elapsed since
-    /// the PY <see cref="ZeroYear"/> anchor). Use for time-delta math.
-    /// For calendar display and for the retail
-    /// <c>SkyDesc::PickCurrentDayGroup</c> LCG seed, use
+    /// the PY <see cref="ZeroYear"/> anchor). Shifted by
+    /// <see cref="OriginOffsetTicks"/> (dat's ZeroTimeOfYear) to match
+    /// retail's TimeOfDay::OnTick extraction. Use for time-delta math.
+    /// For calendar display and for the retail DayGroup LCG seed, use
     /// <see cref="AbsoluteYear"/> which adds the <c>ZeroYear</c> base.
     /// </summary>
     public static int Year(double ticks)
     {
         if (ticks < 0) ticks = 0;
-        return (int)(ticks / YearTicks);
+        double shifted = ticks + OriginOffsetTicks;
+        return (int)(shifted / YearTicks);
     }
 
     /// <summary>
@@ -212,14 +233,18 @@ public static class DerethDateTime
 
     /// <summary>
     /// Day index within the current Derethian year in [0, 359] (360 days
-    /// per year = 12 months × 30). Matches retail's <c>TimeOfDay + 0x68</c>
-    /// field which is <c>floor(withinYearSec / secsPerDay)</c>. Consumed
-    /// by <c>SkyDesc.PickCurrentDayGroup</c> as part of the per-day seed.
+    /// per year = 12 months × 30). Shifted by <see cref="OriginOffsetTicks"/>
+    /// (dat's ZeroTimeOfYear) to match retail's <c>TimeOfDay + 0x68</c>
+    /// field (<c>FUN_005a7510:5304</c>:
+    /// <c>floor(withinYearSec / secsPerDay)</c>). Consumed by
+    /// <c>SkyDesc.PickCurrentDayGroup</c> as part of the per-day seed.
     /// </summary>
     public static int DayOfYear(double ticks)
     {
         if (ticks < 0) ticks = 0;
-        double tYear = ticks - Year(ticks) * YearTicks;
+        double shifted = ticks + OriginOffsetTicks;
+        int year = (int)(shifted / YearTicks);
+        double tYear = shifted - year * YearTicks;
         int d = (int)(tYear / DayTicks);
         if (d < 0) d = 0;
         if (d > 359) d = 359;
diff --git a/src/AcDream.Core/World/SkyDescLoader.cs b/src/AcDream.Core/World/SkyDescLoader.cs
index fde70fc..a75edab 100644
--- a/src/AcDream.Core/World/SkyDescLoader.cs
+++ b/src/AcDream.Core/World/SkyDescLoader.cs
@@ -393,6 +393,40 @@ public static class SkyDescLoader
 
         Console.WriteLine("[sky-dump] ======== BEGIN SkyDesc dump ========");
         Console.WriteLine($"[sky-dump] Region Id={region.Id:X8} Number={region.RegionNumber} Name=\"{region.RegionName}\"");
+
+        // Phase 3f diag — retail TimeOfDay::OnTick uses
+        // GameTime.ZeroTimeOfYear as an additive tick offset before
+        // calendar extraction. Our DerethDateTime currently hardcodes
+        // +7/16 (= 3333.75) for the dayFraction math and IGNORES this
+        // field. If the dat value is anything other than what we assume,
+        // our calendar is skewed from retail's at every moment.
+        var gt = region.GameTime;
+        if (gt is not null)
+        {
+            Console.WriteLine(
+                $"[sky-dump] GameTime ZeroTimeOfYear={gt.ZeroTimeOfYear} ZeroYear={gt.ZeroYear} " +
+                $"DayLength={gt.DayLength} DaysPerYear={gt.DaysPerYear} " +
+                $"YearSpec=\"{gt.YearSpec}\" TimesOfDay.Count={gt.TimesOfDay?.Count ?? 0} " +
+                $"DaysOfWeek.Count={gt.DaysOfWeek?.Count ?? 0} Seasons.Count={gt.Seasons?.Count ?? 0}");
+
+            // Dump every TimeOfDay slot — this is the retail-authoritative
+            // hour boundary table. Anchors our offset math: if retail
+            // TimeOfDay[0].Start != our assumed 0, we have our answer.
+            if (gt.TimesOfDay is not null)
+            {
+                for (int i = 0; i < gt.TimesOfDay.Count; i++)
+                {
+                    var t = gt.TimesOfDay[i];
+                    Console.WriteLine(
+                        $"[sky-dump]   TimeOfDay[{i}] Start={t.Start} IsNight={t.IsNight} Name=\"{t.Name}\"");
+                }
+            }
+        }
+        else
+        {
+            Console.WriteLine("[sky-dump] GameTime: null (no calendar info in this region)");
+        }
+
         Console.WriteLine($"[sky-dump] SkyDesc TickSize={sky.TickSize} LightTickSize={sky.LightTickSize} DayGroups.Count={sky.DayGroups.Count}");
 
         for (int g = 0; g < sky.DayGroups.Count; g++)