diff --git a/src/TheLivingWorld.Api/Generation/WorldGenerationService.cs b/src/TheLivingWorld.Api/Generation/WorldGenerationService.cs
index 4e29d2a..b5b1cf5 100644
--- a/src/TheLivingWorld.Api/Generation/WorldGenerationService.cs
+++ b/src/TheLivingWorld.Api/Generation/WorldGenerationService.cs
@@ -7,6 +7,7 @@ using TheLivingWorld.Api.Storage;
 using TheLivingWorld.Core.Contracts;
 using TheLivingWorld.Core.Export;
 using TheLivingWorld.Core.Geo;
+using TheLivingWorld.Core.Simulation;
 using TheLivingWorld.Core.Worlds;
 using TheLivingWorld.Osm.Import;
 
@@ -46,6 +47,16 @@ public sealed class WorldGenerationService(
         if (!double.IsFinite(request.SizeKm) || request.SizeKm is < MinSizeKm or > MaxSizeKm)
             throw new ArgumentException($"Size must be between {MinSizeKm} and {MaxSizeKm} km.");
 
+        DateTime startGameTime;
+        try
+        {
+            startGameTime = GameTime.ResolveStart(request.StartGameTime);
+        }
+        catch (ArgumentOutOfRangeException ex)
+        {
+            throw new ArgumentException(ex.Message, ex);
+        }
+
         var name = string.IsNullOrWhiteSpace(request.Name)
             ? $"{origin.Latitude:F4}, {origin.Longitude:F4}"
             : request.Name.Trim();
@@ -61,7 +72,7 @@ public sealed class WorldGenerationService(
             Status = WorldStatus.Pending,
             Stage = "Queued",
             CreatedAt = DateTimeOffset.UtcNow,
-            Clock = WorldSimulation.DefaultClock(),
+            Clock = WorldSimulation.DefaultClock(startGameTime),
         };
 
         await _capacityGate.WaitAsync(cancellationToken).ConfigureAwait(false);
diff --git a/src/TheLivingWorld.Api/Simulation/WorldSimulation.cs b/src/TheLivingWorld.Api/Simulation/WorldSimulation.cs
index 83f1aa8..e73f665 100644
--- a/src/TheLivingWorld.Api/Simulation/WorldSimulation.cs
+++ b/src/TheLivingWorld.Api/Simulation/WorldSimulation.cs
@@ -70,9 +70,9 @@ public sealed class WorldSimulation : IDisposable
         return simulation;
     }
 
-    public static WorldClockDto DefaultClock() => new()
+    public static WorldClockDto DefaultClock(DateTime? startGameTime = null) => new()
     {
-        GameTime = GameTime.DefaultStart,
+        GameTime = GameTime.ResolveStart(startGameTime),
         TimeScale = GameTime.MinTimeScale,
         Paused = false,
     };
diff --git a/src/TheLivingWorld.Core/Contracts/WorldContracts.cs b/src/TheLivingWorld.Core/Contracts/WorldContracts.cs
index 0f610ba..921f097 100644
--- a/src/TheLivingWorld.Core/Contracts/WorldContracts.cs
+++ b/src/TheLivingWorld.Core/Contracts/WorldContracts.cs
@@ -22,6 +22,11 @@ public sealed record CreateWorldRequest
 
     /// <summary>Re-download from Overpass even if a cached response for this box exists.</summary>
     public bool ForceRefresh { get; init; }
+
+    /// <summary>
+    /// Naive in-world calendar start. When omitted, defaults to 12 April 2012 06:00.
+    /// </summary>
+    public DateTime? StartGameTime { get; init; }
 }
 
 /// <summary>Response body for <c>GET /api/worlds</c>.</summary>
diff --git a/src/TheLivingWorld.Core/Simulation/GameTime.cs b/src/TheLivingWorld.Core/Simulation/GameTime.cs
index e25816c..9904c65 100644
--- a/src/TheLivingWorld.Core/Simulation/GameTime.cs
+++ b/src/TheLivingWorld.Core/Simulation/GameTime.cs
@@ -28,4 +28,20 @@ public static class GameTime
 
     public static bool IsValidTimeScale(int timeScale) =>
         timeScale is >= MinTimeScale and <= MaxTimeScale;
+
+    /// <summary>
+    /// Normalises a requested start to a naive local calendar instant. Null → <see cref="DefaultStart"/>.
+    /// Rejects values outside a practical game range (year 1–9998).
+    /// </summary>
+    public static DateTime ResolveStart(DateTime? requested)
+    {
+        if (requested is null) return DefaultStart;
+
+        var value = DateTime.SpecifyKind(requested.Value, DateTimeKind.Unspecified);
+        if (value.Year is < 1 or > 9998)
+            throw new ArgumentOutOfRangeException(nameof(requested), "Start game time year must be between 1 and 9998.");
+
+        // Drop sub-minute noise — the clock displays minutes only.
+        return new DateTime(value.Year, value.Month, value.Day, value.Hour, value.Minute, 0, DateTimeKind.Unspecified);
+    }
 }
diff --git a/src/TheLivingWorld.Web/index.html b/src/TheLivingWorld.Web/index.html
index 71e0c81..9e9f26a 100644
--- a/src/TheLivingWorld.Web/index.html
+++ b/src/TheLivingWorld.Web/index.html
@@ -68,6 +68,16 @@
               <input id="field-size" type="range" min="1" max="20" step="1" value="10" />
             </label>
 
+            <label class="field">
+              <span>Start date &amp; time</span>
+              <input
+                id="field-start"
+                type="datetime-local"
+                value="2012-04-12T06:00"
+                required
+              />
+            </label>
+
             <p id="form-hint" class="form__hint" hidden></p>
             <button id="generate-button" type="submit" class="button">Generate world</button>
           </form>
diff --git a/src/TheLivingWorld.Web/src/api/types.ts b/src/TheLivingWorld.Web/src/api/types.ts
index 997a5d4..23a967f 100644
--- a/src/TheLivingWorld.Web/src/api/types.ts
+++ b/src/TheLivingWorld.Web/src/api/types.ts
@@ -48,6 +48,8 @@ export interface CreateWorldRequest {
   longitude: number;
   sizeKm: number;
   forceRefresh?: boolean;
+  /** Naive local game calendar start, e.g. `2012-04-12T06:00:00`. Omits → server default. */
+  startGameTime?: string;
 }
 
 /** [minX, minY, maxX, maxY] in world metres. */
diff --git a/src/TheLivingWorld.Web/src/main.ts b/src/TheLivingWorld.Web/src/main.ts
index 0fde483..2bd24a6 100644
--- a/src/TheLivingWorld.Web/src/main.ts
+++ b/src/TheLivingWorld.Web/src/main.ts
@@ -4,7 +4,12 @@ import type { WorldClock, WorldSummary } from './api/types';
 import { MapView, type MapStatus } from './map/mapView';
 import { THEMES, type ThemeName } from './map/theme';
 import { formatCoordinates, parseCoordinates } from './ui/coordinates';
-import { formatGameTime, formatGameTimeRaw, interpolateGameTime } from './ui/gameTime';
+import {
+  formatGameTime,
+  formatGameTimeRaw,
+  interpolateGameTime,
+  startGameTimeFromInput,
+} from './ui/gameTime';
 
 const LAST_WORLD_KEY = 'the-living-world:last-world';
 const THEME_KEY = 'the-living-world:theme';
@@ -21,6 +26,7 @@ const elements = {
   coords: required<HTMLInputElement>('field-coords'),
   size: required<HTMLInputElement>('field-size'),
   sizeValue: required<HTMLOutputElement>('field-size-value'),
+  start: required<HTMLInputElement>('field-start'),
   generate: required<HTMLButtonElement>('generate-button'),
   formHint: required<HTMLElement>('form-hint'),
   useLocation: required<HTMLButtonElement>('use-location'),
@@ -446,6 +452,13 @@ async function generate(event: SubmitEvent): Promise<void> {
     return;
   }
 
+  const startGameTime = startGameTimeFromInput(elements.start.value);
+  if (!startGameTime) {
+    setStatus('Pick a start date and time for the in-world calendar.', 'error');
+    elements.start.focus();
+    return;
+  }
+
   const sizeKm = Number(elements.size.value);
   generating = true;
   updateGenerateEnabled();
@@ -457,6 +470,7 @@ async function generate(event: SubmitEvent): Promise<void> {
       latitude: location.latitude,
       longitude: location.longitude,
       sizeKm,
+      startGameTime,
     });
 
     await refreshWorldList();
diff --git a/src/TheLivingWorld.Web/src/styles.css b/src/TheLivingWorld.Web/src/styles.css
index 2de7bf1..9797ea2 100644
--- a/src/TheLivingWorld.Web/src/styles.css
+++ b/src/TheLivingWorld.Web/src/styles.css
@@ -323,7 +323,8 @@ body {
 }
 
 .field input[type='text'],
-.field input[type='number'] {
+.field input[type='number'],
+.field input[type='datetime-local'] {
   padding: 7px 9px;
   font: inherit;
   font-size: 13px;
diff --git a/src/TheLivingWorld.Web/src/ui/gameTime.test.ts b/src/TheLivingWorld.Web/src/ui/gameTime.test.ts
index 27dc64f..9aa2a51 100644
--- a/src/TheLivingWorld.Web/src/ui/gameTime.test.ts
+++ b/src/TheLivingWorld.Web/src/ui/gameTime.test.ts
@@ -5,6 +5,8 @@ import {
   GAME_MINUTES_PER_REAL_SECOND,
   interpolateGameTime,
   parseGameTime,
+  startGameTimeFromInput,
+  DEFAULT_START_INPUT,
 } from './gameTime';
 
 describe('parseGameTime', () => {
@@ -64,3 +66,12 @@ describe('interpolateGameTime', () => {
     expect(formatGameTime(date!)).toBe('12 April 2012 · 06:10');
   });
 });
+
+describe('startGameTimeFromInput', () => {
+  it('converts a datetime-local value to naive ISO', () => {
+    expect(startGameTimeFromInput('2012-04-12T06:00')).toBe('2012-04-12T06:00:00');
+    expect(startGameTimeFromInput(DEFAULT_START_INPUT)).toBe('2012-04-12T06:00:00');
+    expect(startGameTimeFromInput('')).toBeNull();
+    expect(startGameTimeFromInput('not-a-date')).toBeNull();
+  });
+});
diff --git a/src/TheLivingWorld.Web/src/ui/gameTime.ts b/src/TheLivingWorld.Web/src/ui/gameTime.ts
index de659bf..26d26a4 100644
--- a/src/TheLivingWorld.Web/src/ui/gameTime.ts
+++ b/src/TheLivingWorld.Web/src/ui/gameTime.ts
@@ -68,3 +68,39 @@ export function interpolateGameTime(
   const gameMs = (elapsedRealMs / 1000) * GAME_MINUTES_PER_REAL_SECOND * scale * 60_000;
   return new Date(base.getTime() + gameMs);
 }
+
+/** Default start shown in the create form (`datetime-local` value). */
+export const DEFAULT_START_INPUT = '2012-04-12T06:00';
+
+/**
+ * Converts a `datetime-local` value (`YYYY-MM-DDTHH:mm`) into the naive ISO string the API expects.
+ * Returns null when the field is empty or malformed.
+ */
+export function startGameTimeFromInput(raw: string): string | null {
+  const trimmed = raw.trim();
+  if (!trimmed) return null;
+
+  const match = /^(\d{4})-(\d{2})-(\d{2})T(\d{2}):(\d{2})(?::(\d{2}))?$/.exec(trimmed);
+  if (!match) return null;
+
+  const year = Number(match[1]);
+  const month = Number(match[2]);
+  const day = Number(match[3]);
+  const hour = Number(match[4]);
+  const minute = Number(match[5]);
+  const second = Number(match[6] ?? '0');
+
+  const date = new Date(year, month - 1, day, hour, minute, second);
+  if (
+    date.getFullYear() !== year
+    || date.getMonth() !== month - 1
+    || date.getDate() !== day
+    || date.getHours() !== hour
+    || date.getMinutes() !== minute
+  ) {
+    return null;
+  }
+
+  const pad = (n: number) => String(n).padStart(2, '0');
+  return `${year}-${pad(month)}-${pad(day)}T${pad(hour)}:${pad(minute)}:00`;
+}
diff --git a/tests/TheLivingWorld.Tests/GameTimeTests.cs b/tests/TheLivingWorld.Tests/GameTimeTests.cs
index e4c68ee..15f1990 100644
--- a/tests/TheLivingWorld.Tests/GameTimeTests.cs
+++ b/tests/TheLivingWorld.Tests/GameTimeTests.cs
@@ -58,4 +58,20 @@ public sealed class GameTimeTests
     {
         Assert.Equal(expected, GameTime.IsValidTimeScale(scale));
     }
+
+    [Fact]
+    public void ResolveStart_defaults_and_normalises()
+    {
+        Assert.Equal(GameTime.DefaultStart, GameTime.ResolveStart(null));
+        Assert.Equal(
+            new DateTime(1999, 12, 31, 23, 45, 0, DateTimeKind.Unspecified),
+            GameTime.ResolveStart(new DateTime(1999, 12, 31, 23, 45, 59, DateTimeKind.Utc)));
+    }
+
+    [Fact]
+    public void ResolveStart_rejects_out_of_range_years()
+    {
+        Assert.Throws<ArgumentOutOfRangeException>(() =>
+            GameTime.ResolveStart(new DateTime(10000, 1, 1)));
+    }
 }
diff --git a/tests/TheLivingWorld.Tests/WorldGenerationServiceTests.cs b/tests/TheLivingWorld.Tests/WorldGenerationServiceTests.cs
index 1a94591..52f4c8d 100644
--- a/tests/TheLivingWorld.Tests/WorldGenerationServiceTests.cs
+++ b/tests/TheLivingWorld.Tests/WorldGenerationServiceTests.cs
@@ -45,6 +45,29 @@ public sealed class WorldGenerationServiceTests : IDisposable
         Assert.Equal("Taken", Assert.Single(await _store.ListAsync()).Name);
     }
 
+    [Fact]
+    public async Task StartAsync_uses_requested_start_game_time_on_the_pending_clock()
+    {
+        using var service = CreateService(maxConcurrentWorlds: 2);
+        var start = new DateTime(1995, 6, 15, 8, 30, 0, DateTimeKind.Unspecified);
+
+        var summary = await service.StartAsync(new CreateWorldRequest
+        {
+            Name = "Custom clock",
+            Latitude = 31.8966010,
+            Longitude = -100.4858591,
+            SizeKm = 5,
+            StartGameTime = start,
+        }, CancellationToken.None);
+
+        Assert.NotNull(summary.Clock);
+        Assert.Equal(start, summary.Clock.GameTime);
+
+        var stored = await _store.GetSummaryAsync(summary.Id);
+        Assert.NotNull(stored?.Clock);
+        Assert.Equal(start, stored.Clock.GameTime);
+    }
+
     private WorldGenerationService CreateService(int maxConcurrentWorlds)
     {
         // The capacity check runs before any Overpass work, so this generator is never invoked by the