diff --git a/SVSim.EmulatedEntrypoint/Services/JstPeriod.cs b/SVSim.EmulatedEntrypoint/Services/JstPeriod.cs
new file mode 100644
index 0000000..572fadb
--- /dev/null
+++ b/SVSim.EmulatedEntrypoint/Services/JstPeriod.cs
@@ -0,0 +1,53 @@
+using System.Globalization;
+
+namespace SVSim.EmulatedEntrypoint.Services;
+
+/// <summary>
+/// JST-anchored period bucketing for ViewerEventCounters. Day/week/month boundaries are
+/// 02:00 JST (matching the real-game daily reset). Pure functions, no dependencies.
+/// </summary>
+public static class JstPeriod
+{
+    private static readonly TimeSpan Jst = TimeSpan.FromHours(9);
+    private const string DayPrefix   = "day:";
+    private const string WeekPrefix  = "week:";
+    private const string MonthPrefix = "month:";
+    public const string AllTime = "all-time";
+
+    /// <summary>
+    /// Converts the given instant to a JST-anchored "effective date" by:
+    /// 1. Shifting to JST (+09:00)
+    /// 2. Subtracting 2 hours so anything before 02:00 JST belongs to the previous day
+    /// </summary>
+    private static DateTime EffectiveJstDate(DateTimeOffset utcOrAny)
+    {
+        var jst = utcOrAny.ToOffset(Jst);
+        return jst.AddHours(-2).Date;
+    }
+
+    public static string DayKey(DateTimeOffset when)
+    {
+        var d = EffectiveJstDate(when);
+        return $"{DayPrefix}{d:yyyy-MM-dd}";
+    }
+
+    public static string WeekKey(DateTimeOffset when)
+    {
+        var d = EffectiveJstDate(when);
+        var iso = ISOWeek.GetWeekOfYear(d);
+        var year = ISOWeek.GetYear(d);
+        return $"{WeekPrefix}{year:D4}-W{iso:D2}";
+    }
+
+    public static string MonthKey(DateTimeOffset when)
+    {
+        var d = EffectiveJstDate(when);
+        return $"{MonthPrefix}{d:yyyy-MM}";
+    }
+
+    /// <summary>Returns [day, week, month, all-time] keys for the given instant.</summary>
+    public static IReadOnlyList<string> AllPeriods(DateTimeOffset when) => new[]
+    {
+        DayKey(when), WeekKey(when), MonthKey(when), AllTime,
+    };
+}
diff --git a/SVSim.UnitTests/Services/JstPeriodTests.cs b/SVSim.UnitTests/Services/JstPeriodTests.cs
new file mode 100644
index 0000000..f243673
--- /dev/null
+++ b/SVSim.UnitTests/Services/JstPeriodTests.cs
@@ -0,0 +1,52 @@
+namespace SVSim.UnitTests.Services;
+
+public class JstPeriodTests
+{
+    private static DateTimeOffset Jst(int y, int m, int d, int h, int min, int s) =>
+        new DateTimeOffset(y, m, d, h, min, s, TimeSpan.FromHours(9));
+
+    [Test]
+    public void DayKey_uses_jst_with_02_00_anchor()
+    {
+        // 01:59:59 JST on May 27 belongs to the May 26 "day".
+        Assert.That(SVSim.EmulatedEntrypoint.Services.JstPeriod.DayKey(Jst(2026, 5, 27, 1, 59, 59)),
+            Is.EqualTo("day:2026-05-26"));
+
+        // 02:00:00 JST on May 27 belongs to the May 27 "day".
+        Assert.That(SVSim.EmulatedEntrypoint.Services.JstPeriod.DayKey(Jst(2026, 5, 27, 2, 0, 0)),
+            Is.EqualTo("day:2026-05-27"));
+    }
+
+    [Test]
+    public void WeekKey_is_iso8601_monday_anchored()
+    {
+        // 2026-05-25 (Monday) 02:00 JST → ISO week 2026-W22.
+        Assert.That(SVSim.EmulatedEntrypoint.Services.JstPeriod.WeekKey(Jst(2026, 5, 25, 2, 0, 0)),
+            Is.EqualTo("week:2026-W22"));
+
+        // The previous Sunday's late evening still belongs to W21 (week reset is Monday 02:00 JST).
+        Assert.That(SVSim.EmulatedEntrypoint.Services.JstPeriod.WeekKey(Jst(2026, 5, 25, 1, 59, 59)),
+            Is.EqualTo("week:2026-W21"));
+    }
+
+    [Test]
+    public void MonthKey_uses_jst_day_anchor_for_month_rollover()
+    {
+        // 01:59 JST on June 1 still belongs to May (day-boundary is 02:00 JST).
+        Assert.That(SVSim.EmulatedEntrypoint.Services.JstPeriod.MonthKey(Jst(2026, 6, 1, 1, 59, 0)),
+            Is.EqualTo("month:2026-05"));
+
+        Assert.That(SVSim.EmulatedEntrypoint.Services.JstPeriod.MonthKey(Jst(2026, 6, 1, 2, 0, 0)),
+            Is.EqualTo("month:2026-06"));
+    }
+
+    [Test]
+    public void AllPeriods_returns_day_week_month_all_time()
+    {
+        var t = Jst(2026, 5, 27, 12, 0, 0);
+        var periods = SVSim.EmulatedEntrypoint.Services.JstPeriod.AllPeriods(t);
+        Assert.That(periods, Is.EquivalentTo(new[] {
+            "day:2026-05-27", "week:2026-W22", "month:2026-05", "all-time",
+        }));
+    }
+}