fix(fake-timers): convert Date to milliseconds before passing to @sinonjs/fake-timers (#16029)

Gdhanush-13 · web-flow · commit 3c483bfe769c · 2026-05-01T14:26:26.000+02:00
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -10,6 +10,7 @@
 ### Fixes
 
 - `[expect-utils]` Fix `toStrictEqual` failing on `structuredClone` results due to cross-realm constructor mismatch ([#14011](https://github.com/jestjs/jest/issues/14011))
+- `[fake-timers)]` Convert `Date` to milliseconds before passing to `@sinonjs/fake-timers` ([#16029](https://github.com/jestjs/jest/pull/16029))
 - `[jest-runtime]` Improve CJS-from-ESM interop: `__esModule`/Babel default unwrap, broader named-export coverage, and shared CJS singleton across importers ([#16050](https://github.com/jestjs/jest/pull/16050))
 - `[jest-runtime]` Load `.js` files with ESM syntax but no `"type":"module"` marker as native ESM ([#16050](https://github.com/jestjs/jest/pull/16050))
 - `[jest-runtime]` Fix deadlocks and double-evaluation in concurrent ESM and wasm imports ([#16050](https://github.com/jestjs/jest/pull/16050))
diff --git a/packages/jest-fake-timers/src/__tests__/sinon-integration.test.ts b/packages/jest-fake-timers/src/__tests__/sinon-integration.test.ts
@@ -128,7 +128,7 @@ describe('`@sinonjs/fake-timers` integration', () => {
     expect(mockInstall).toHaveBeenCalledWith({
       advanceTimeDelta: 40,
       loopLimit: 2000,
-      now: new Date('1995-12-17'),
+      now: new Date('1995-12-17').getTime(),
       shouldAdvanceTime: true,
       shouldClearNativeTimers: true,
       toFake: [
diff --git a/packages/jest-fake-timers/src/modernFakeTimers.ts b/packages/jest-fake-timers/src/modernFakeTimers.ts
@@ -151,7 +151,7 @@ export default class FakeTimers {
 
   setSystemTime(now?: number | Date): void {
     if (this._checkFakeTimers()) {
-      this._clock.setSystemTime(now);
+      this._clock.setSystemTime(now instanceof Date ? now.getTime() : now);
     }
   }
 
@@ -226,7 +226,10 @@ export default class FakeTimers {
     return {
       advanceTimeDelta,
       loopLimit: fakeTimersConfig.timerLimit || 100_000,
-      now: fakeTimersConfig.now ?? Date.now(),
+      now:
+        fakeTimersConfig.now instanceof Date
+          ? fakeTimersConfig.now.getTime()
+          : (fakeTimersConfig.now ?? Date.now()),
       shouldAdvanceTime: Boolean(fakeTimersConfig.advanceTimers),
       shouldClearNativeTimers: true,
       toFake: [...toFake],
