diff --git a/src/sim/calendar.ts b/src/sim/calendar.ts
new file mode 100644
index 0000000..f76111e
--- /dev/null
+++ b/src/sim/calendar.ts
@@ -0,0 +1,54 @@
+export interface GameDate {
+  year: number;
+  month: number; // 1-12
+  day: number;
+}
+
+const MS_PER_DAY = 86_400_000;
+
+export const START_DATE: GameDate = { year: 1990, month: 9, day: 28 };
+export const EURO_DATE: GameDate = { year: 2002, month: 1, day: 1 };
+
+function toUTC(d: GameDate): number {
+  return Date.UTC(d.year, d.month - 1, d.day);
+}
+
+function fromUTC(ms: number): GameDate {
+  const js = new Date(ms);
+  return { year: js.getUTCFullYear(), month: js.getUTCMonth() + 1, day: js.getUTCDate() };
+}
+
+export function addDays(d: GameDate, n: number): GameDate {
+  return fromUTC(toUTC(d) + n * MS_PER_DAY);
+}
+
+export function daysBetween(a: GameDate, b: GameDate): number {
+  return Math.round((toUTC(b) - toUTC(a)) / MS_PER_DAY);
+}
+
+export function compareDates(a: GameDate, b: GameDate): number {
+  return toUTC(a) - toUTC(b);
+}
+
+export function isSunday(d: GameDate): boolean {
+  return new Date(toUTC(d)).getUTCDay() === 0;
+}
+
+export function isLastDayOfMonth(d: GameDate): boolean {
+  return addDays(d, 1).month !== d.month;
+}
+
+export function isEuroEra(d: GameDate): boolean {
+  return compareDates(d, EURO_DATE) >= 0;
+}
+
+export function dateToISO(d: GameDate): string {
+  const mm = String(d.month).padStart(2, '0');
+  const dd = String(d.day).padStart(2, '0');
+  return `${d.year}-${mm}-${dd}`;
+}
+
+export function parseISO(s: string): GameDate {
+  const [year, month, day] = s.split('-').map(Number);
+  return { year, month, day };
+}
diff --git a/tests/sim/calendar.test.ts b/tests/sim/calendar.test.ts
new file mode 100644
index 0000000..4a7f4ee
--- /dev/null
+++ b/tests/sim/calendar.test.ts
@@ -0,0 +1,44 @@
+import { describe, it, expect } from 'vitest';
+import {
+  addDays, compareDates, dateToISO, daysBetween, isEuroEra,
+  isLastDayOfMonth, isSunday, parseISO, START_DATE,
+} from '../../src/sim/calendar';
+
+describe('calendar', () => {
+  it('starts on the European Game Boy launch (Friday 28/09/1990)', () => {
+    expect(START_DATE).toEqual({ year: 1990, month: 9, day: 28 });
+    expect(isSunday(START_DATE)).toBe(false);
+  });
+
+  it('detects Sundays', () => {
+    expect(isSunday({ year: 1990, month: 9, day: 30 })).toBe(true);
+    expect(isSunday({ year: 1990, month: 9, day: 29 })).toBe(false);
+  });
+
+  it('adds days across month and year boundaries', () => {
+    expect(addDays({ year: 1990, month: 12, day: 31 }, 1)).toEqual({ year: 1991, month: 1, day: 1 });
+    expect(addDays({ year: 1990, month: 9, day: 28 }, 3)).toEqual({ year: 1990, month: 10, day: 1 });
+  });
+
+  it('detects the last day of a month, including leap years', () => {
+    expect(isLastDayOfMonth({ year: 1992, month: 2, day: 29 })).toBe(true);
+    expect(isLastDayOfMonth({ year: 1992, month: 2, day: 28 })).toBe(false);
+    expect(isLastDayOfMonth({ year: 1990, month: 9, day: 30 })).toBe(true);
+  });
+
+  it('switches to the euro on 01/01/2002', () => {
+    expect(isEuroEra({ year: 2001, month: 12, day: 31 })).toBe(false);
+    expect(isEuroEra({ year: 2002, month: 1, day: 1 })).toBe(true);
+  });
+
+  it('computes days between dates', () => {
+    expect(daysBetween({ year: 1990, month: 9, day: 28 }, { year: 1990, month: 10, day: 28 })).toBe(30);
+    expect(daysBetween({ year: 1990, month: 10, day: 28 }, { year: 1990, month: 9, day: 28 })).toBe(-30);
+  });
+
+  it('round-trips ISO strings and compares dates', () => {
+    expect(dateToISO(parseISO('1990-09-28'))).toBe('1990-09-28');
+    expect(compareDates(parseISO('1990-09-28'), parseISO('1991-01-01'))).toBeLessThan(0);
+    expect(compareDates(parseISO('1991-01-01'), parseISO('1991-01-01'))).toBe(0);
+  });
+});