jestjs/jest · error · TypeError

`jest.setSystemTime()` is not available when using legacy fa

Error message

`jest.setSystemTime()` is not available when using legacy fake timers.

What it means

TypeError thrown by `jest.setSystemTime()` when legacy fake timers are active. Setting the fake system clock is a modern-fake-timers-only feature; the legacy implementation does not expose `setSystemTime`.

Source

Thrown at packages/jest-runtime/src/internals/JestGlobals.ts:413

      runOnlyPendingTimersAsync: async () => {
        const fakeTimers = _getFakeTimers();

        if (fakeTimers === this.environment.fakeTimersModern) {
          await fakeTimers.runOnlyPendingTimersAsync();
        } else {
          throw new TypeError(
            '`jest.runOnlyPendingTimersAsync()` is not available when using legacy fake timers.',
          );
        }
      },
      setMock: (moduleName, mock) => setMockFactory(moduleName, () => mock),
      setSystemTime: now => {
        const fakeTimers = _getFakeTimers();

        if (fakeTimers === this.environment.fakeTimersModern) {
          fakeTimers.setSystemTime(now);
        } else {
          throw new TypeError(
            '`jest.setSystemTime()` is not available when using legacy fake timers.',
          );
        }
      },
      setTimeout,
      setTimerTickMode: (
        mode:
          | {mode: 'manual' | 'nextAsync'}
          | {mode: 'interval'; delta?: number},
      ) => {
        const fakeTimers = _getFakeTimers();
        if (fakeTimers === this.environment.fakeTimersModern) {
          fakeTimers.setTimerTickMode(mode);
        } else {
          throw new TypeError(
            '`jest.setTimerTickMode()` is not available when using legacy fake timers.',
          );
        }

View on GitHub (pinned to f49721c78e)

Solutions

1. Switch to modern fake timers: `jest.useFakeTimers()` (default).
2. If legacy timers are required, mock `Date` directly with `jest.spyOn(global, 'Date')` or use a library like `mockdate`, though migrating to modern timers is strongly preferred.

Example fix

// before
jest.useFakeTimers({ legacyFakeTimers: true });
jest.setSystemTime(new Date('2025-01-01')); // throws

// after
jest.useFakeTimers();
jest.setSystemTime(new Date('2025-01-01'));

// Modern timers support setSystemTime; legacy does not
jest.useFakeTimers(); // modern
jest.setSystemTime(new Date('2025-01-01'));

When it happens

Trigger: Calling `jest.setSystemTime(now)` while `legacyFakeTimers: true` is set.

Common situations: Time-dependent tests (e.g. asserting on `new Date()` output) where the project defaults to legacy timers; time-travel tests for date libraries.

Related errors

• `jest.getRealSystemTime()` is not available when using legac
• `jest.advanceTimersByTimeAsync()` is not available when usin
• `jest.advanceTimersToNextFrame()` is not available when usin
• `jest.advanceTimersToNextTimerAsync()` is not available when
• `jest.runAllTimersAsync()` is not available when using legac

AI-assisted analysis of jestjs/jest@f49721c78e (2026-08-03). Data as JSON: /data/errors/23cb0ebaac7e80dd.json. Report an issue: GitHub.