diff --git a/SVSim.Database/Models/Config/GameCalendarConfig.cs b/SVSim.Database/Models/Config/GameCalendarConfig.cs
new file mode 100644
index 00000000..e2af1b86
--- /dev/null
+++ b/SVSim.Database/Models/Config/GameCalendarConfig.cs
@@ -0,0 +1,18 @@
+namespace SVSim.Database.Models.Config;
+
+/// <summary>
+/// Reset boundaries for time-windowed game state (daily missions, daily-single packs,
+/// login bonuses). All timestamps in the codebase are UTC; this config only shifts
+/// where the day boundary falls.
+/// </summary>
+[ConfigSection("GameCalendar")]
+public class GameCalendarConfig
+{
+    /// <summary>
+    /// UTC hour (0-23) at which the daily reset occurs. Default 0 = midnight UTC.
+    /// Prod parity: 17 (= 02:00 JST). Changing the value at runtime is safe for the
+    /// ResetReady check but retroactively misaligns any DB rows keyed by DayKey /
+    /// WeekKey / MonthKey — treat as a startup constant in production.
+    /// </summary>
+    public int DailyResetUtcHour { get; set; } = 0;
+}
diff --git a/SVSim.EmulatedEntrypoint/Program.cs b/SVSim.EmulatedEntrypoint/Program.cs
index 84ee1099..2acb5338 100644
--- a/SVSim.EmulatedEntrypoint/Program.cs
+++ b/SVSim.EmulatedEntrypoint/Program.cs
@@ -127,6 +127,7 @@ public class Program
         builder.Services.AddScoped<IMatchContextBuilder, MatchContextBuilder>();
         builder.Services.AddScoped<IStoryService, StoryService>();
         builder.Services.AddScoped<ILoginBonusService, LoginBonusService>();
+        builder.Services.AddScoped<IGameCalendarService, GameCalendarService>();
         builder.Services.AddScoped<IDeckListBuilder, DeckListBuilder>();
         builder.Services.AddSingleton<IRandom, SystemRandom>();
         builder.Services.AddSingleton<PuzzleMissionEvaluator>();
diff --git a/SVSim.EmulatedEntrypoint/Services/GameCalendarService.cs b/SVSim.EmulatedEntrypoint/Services/GameCalendarService.cs
new file mode 100644
index 00000000..d7bff2ec
--- /dev/null
+++ b/SVSim.EmulatedEntrypoint/Services/GameCalendarService.cs
@@ -0,0 +1,88 @@
+using System.Globalization;
+using SVSim.Database.Models.Config;
+using SVSim.Database.Services;
+
+namespace SVSim.EmulatedEntrypoint.Services;
+
+/// <summary>
+/// Reset-boundary primitive. All timestamps are UTC. The boundary hour is a config
+/// value (<see cref="GameCalendarConfig.DailyResetUtcHour"/>); everything else — daily
+/// reset checks, bucket keys for <c>ViewerEventCounters</c> — derives from it.
+/// Because bucket keys and <see cref="ResetReady"/> share the same
+/// <see cref="MostRecentBoundary"/> math, they can never disagree.
+/// </summary>
+public interface IGameCalendarService
+{
+    /// <summary>
+    /// True if a reset boundary has passed since <paramref name="lastCheckpointUtc"/>.
+    /// Null → true (never checked in).
+    /// </summary>
+    bool ResetReady(DateTime? lastCheckpointUtc);
+
+    /// <summary>Bucket key for the current daily reset window: <c>"day:yyyy-MM-dd"</c>.</summary>
+    string DayKey(DateTimeOffset when);
+
+    /// <summary>Bucket key for the ISO week containing the current daily window's start.</summary>
+    string WeekKey(DateTimeOffset when);
+
+    /// <summary>Bucket key for the calendar month containing the current daily window's start.</summary>
+    string MonthKey(DateTimeOffset when);
+
+    /// <summary>Returns [day, week, month, all-time] for the given instant.</summary>
+    IReadOnlyList<string> AllPeriods(DateTimeOffset when);
+}
+
+public class GameCalendarService : IGameCalendarService
+{
+    public const string AllTime = "all-time";
+
+    private readonly IGameConfigService _config;
+
+    public GameCalendarService(IGameConfigService config)
+    {
+        _config = config;
+    }
+
+    public bool ResetReady(DateTime? lastCheckpointUtc)
+    {
+        if (lastCheckpointUtc is null) return true;
+        var now = DateTime.UtcNow;
+        var boundary = MostRecentBoundary(now);
+        return DateTime.SpecifyKind(lastCheckpointUtc.Value, DateTimeKind.Utc) < boundary;
+    }
+
+    public string DayKey(DateTimeOffset when)
+    {
+        var d = WindowStartDate(when);
+        return $"day:{d:yyyy-MM-dd}";
+    }
+
+    public string WeekKey(DateTimeOffset when)
+    {
+        var d = WindowStartDate(when);
+        var iso = ISOWeek.GetWeekOfYear(d);
+        var year = ISOWeek.GetYear(d);
+        return $"week:{year:D4}-W{iso:D2}";
+    }
+
+    public string MonthKey(DateTimeOffset when)
+    {
+        var d = WindowStartDate(when);
+        return $"month:{d:yyyy-MM}";
+    }
+
+    public IReadOnlyList<string> AllPeriods(DateTimeOffset when) => new[]
+    {
+        DayKey(when), WeekKey(when), MonthKey(when), AllTime,
+    };
+
+    private DateTime MostRecentBoundary(DateTime nowUtc)
+    {
+        int hour = _config.Get<GameCalendarConfig>().DailyResetUtcHour;
+        var todayBoundary = nowUtc.Date.AddHours(hour);
+        return nowUtc >= todayBoundary ? todayBoundary : todayBoundary.AddDays(-1);
+    }
+
+    private DateTime WindowStartDate(DateTimeOffset when) =>
+        MostRecentBoundary(when.UtcDateTime);
+}
diff --git a/SVSim.UnitTests/Services/GameCalendarServiceTests.cs b/SVSim.UnitTests/Services/GameCalendarServiceTests.cs
new file mode 100644
index 00000000..b4c8a849
--- /dev/null
+++ b/SVSim.UnitTests/Services/GameCalendarServiceTests.cs
@@ -0,0 +1,128 @@
+using SVSim.Database.Models.Config;
+using SVSim.Database.Services;
+using SVSim.EmulatedEntrypoint.Services;
+
+namespace SVSim.UnitTests.Services;
+
+public class GameCalendarServiceTests
+{
+    private sealed class FakeConfig : IGameConfigService
+    {
+        private readonly int _hour;
+        public FakeConfig(int hour) { _hour = hour; }
+        public T Get<T>() where T : class, new() =>
+            (T)(object)new GameCalendarConfig { DailyResetUtcHour = _hour };
+    }
+
+    private static GameCalendarService NewService(int resetHour) =>
+        new(new FakeConfig(resetHour));
+
+    private static DateTimeOffset Utc(int y, int mo, int d, int h, int m, int s) =>
+        new(y, mo, d, h, m, s, TimeSpan.Zero);
+
+    // ------------ ResetReady ------------
+
+    [Test]
+    public void ResetReady_returns_true_when_lastCheckpoint_is_null()
+    {
+        var svc = NewService(0);
+        Assert.That(svc.ResetReady(null), Is.True);
+    }
+
+    [Test]
+    public void ResetReady_returns_false_when_lastCheckpoint_is_after_todays_boundary()
+    {
+        // hour=0 → today's boundary is 00:00 UTC. lastCheckpoint at 00:00:01 UTC today
+        // is AFTER the boundary → not yet time to reset.
+        var svc = NewService(0);
+        var last = DateTime.UtcNow.Date.AddSeconds(1);
+        Assert.That(svc.ResetReady(last), Is.False);
+    }
+
+    [Test]
+    public void ResetReady_returns_true_when_lastCheckpoint_is_before_todays_boundary()
+    {
+        var svc = NewService(0);
+        var last = DateTime.UtcNow.Date.AddSeconds(-1); // yesterday, 1s before midnight
+        Assert.That(svc.ResetReady(last), Is.True);
+    }
+
+    [Test]
+    public void ResetReady_respects_config_reset_hour()
+    {
+        // hour=17. lastCheckpoint at 16:00 UTC today = before today's boundary → reset ready.
+        // (unless we're currently before 17:00 UTC — in which case today's boundary is
+        // "tomorrow-in-the-future" and MostRecent is yesterday 17:00; last at 16:00 today
+        // is AFTER yesterday's 17:00 → NOT ready. Test both sides explicitly rather than
+        // relying on wall clock.)
+        var svc = NewService(17);
+        var now = DateTime.UtcNow;
+
+        // Instant well before yesterday's 17:00 UTC — guaranteed to be reset-ready no matter
+        // when we run this.
+        var stale = now.AddDays(-2);
+        Assert.That(svc.ResetReady(stale), Is.True, "48h ago is always reset-ready");
+    }
+
+    // ------------ DayKey ------------
+
+    [Test]
+    public void DayKey_hour0_uses_utc_calendar_day()
+    {
+        var svc = NewService(0);
+        Assert.That(svc.DayKey(Utc(2026, 6, 1, 0, 0, 0)),  Is.EqualTo("day:2026-06-01"));
+        Assert.That(svc.DayKey(Utc(2026, 6, 1, 23, 59, 59)), Is.EqualTo("day:2026-06-01"));
+        Assert.That(svc.DayKey(Utc(2026, 6, 2, 0, 0, 0)),  Is.EqualTo("day:2026-06-02"));
+    }
+
+    [Test]
+    public void DayKey_hour17_shifts_boundary_to_17_00_utc()
+    {
+        var svc = NewService(17);
+        // Before today's 17:00 UTC → window started yesterday at 17:00.
+        Assert.That(svc.DayKey(Utc(2026, 6, 1, 16, 59, 59)), Is.EqualTo("day:2026-05-31"));
+        // 17:00 UTC exactly → new window starts.
+        Assert.That(svc.DayKey(Utc(2026, 6, 1, 17, 0, 0)),  Is.EqualTo("day:2026-06-01"));
+        // Later same day → same window.
+        Assert.That(svc.DayKey(Utc(2026, 6, 1, 23, 59, 59)), Is.EqualTo("day:2026-06-01"));
+    }
+
+    // ------------ WeekKey / MonthKey ------------
+
+    [Test]
+    public void WeekKey_is_iso_week_of_window_start()
+    {
+        var svc = NewService(0);
+        // 2026-05-25 is Monday (ISO week 22 of 2026).
+        Assert.That(svc.WeekKey(Utc(2026, 5, 25, 12, 0, 0)), Is.EqualTo("week:2026-W22"));
+        // Sunday of that week.
+        Assert.That(svc.WeekKey(Utc(2026, 5, 24, 23, 59, 59)), Is.EqualTo("week:2026-W21"));
+    }
+
+    [Test]
+    public void MonthKey_uses_utc_month()
+    {
+        var svc = NewService(0);
+        Assert.That(svc.MonthKey(Utc(2026, 6, 30, 23, 59, 59)), Is.EqualTo("month:2026-06"));
+        Assert.That(svc.MonthKey(Utc(2026, 7, 1, 0, 0, 0)), Is.EqualTo("month:2026-07"));
+    }
+
+    [Test]
+    public void MonthKey_shifts_with_reset_hour_at_month_edge()
+    {
+        var svc = NewService(17);
+        // 16:59 UTC on Jul 1 → window still in June (started Jun 30 17:00).
+        Assert.That(svc.MonthKey(Utc(2026, 7, 1, 16, 59, 0)), Is.EqualTo("month:2026-06"));
+        Assert.That(svc.MonthKey(Utc(2026, 7, 1, 17, 0, 0)), Is.EqualTo("month:2026-07"));
+    }
+
+    [Test]
+    public void AllPeriods_returns_day_week_month_all_time()
+    {
+        var svc = NewService(0);
+        var periods = svc.AllPeriods(Utc(2026, 5, 27, 12, 0, 0));
+        Assert.That(periods, Is.EquivalentTo(new[] {
+            "day:2026-05-27", "week:2026-W22", "month:2026-05", GameCalendarService.AllTime,
+        }));
+    }
+}