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date instanceof Date returns false for dates created before vi.setSystemTime() #9034
Description
Describe the bug
After calling vi.setSystemTime(), date instanceof Date appears to return true only if the date was created after the call. If the date was created before vi.setSystemTime(), the result is false.
Is this expected?
Reproduction
pnpm installpnpm test
example.test.ts contains a test reproducing the issue:
it('should preserve date instanceof after changing the system time', async () => {
const dateBeforeSettingSystemTime = new Date('2025-11-15T08:00:00.000Z');
expect(dateBeforeSettingSystemTime).toBeInstanceOf(Date); // ok
vi.setSystemTime(dateBeforeSettingSystemTime);
const dateAfterSettingSystemTime = new Date();
expect(dateAfterSettingSystemTime).toEqual(dateBeforeSettingSystemTime); // system time set
expect(dateAfterSettingSystemTime).toBeInstanceOf(Date); // ok
expect(dateBeforeSettingSystemTime).toBeInstanceOf(Date); // fails?
}); FAIL example.test.ts > should preserve date instanceof after changing the system time
AssertionError: expected 2025-11-15T08:00:00.000Z to be an instance of MockDate
❯ example.test.ts:13:39
11|
12| expect(dateAfterSettingSystemTime).toBeInstanceOf(Date);
13| expect(dateBeforeSettingSystemTime).toBeInstanceOf(Date);
| ^
14| });
15|
System Info
System:
OS: Linux 6.8 Ubuntu 24.04.3 LTS 24.04.3 LTS (Noble Numbat)
CPU: (12) x64 Intel(R) Core(TM) i5-10400F CPU @ 2.90GHz
Memory: 5.76 GB / 15.54 GB
Container: Yes
Shell: 5.9 - /usr/bin/zsh
Binaries:
Node: 24.11.1 - /usr/bin/node
Yarn: 1.22.22 - /home/diegoaquino/.n/bin/yarn
npm: 11.6.2 - /home/diegoaquino/.n/bin/npm
pnpm: 10.13.1 - /home/diegoaquino/.n/bin/pnpm
bun: 1.3.0 - /home/diegoaquino/.bun/bin/bun
Browsers:
Chrome: 142.0.7444.162
Firefox: 145.0
Firefox Developer Edition: 145.0
npmPackages:
vitest: ^4.0.9 => 4.0.9
``Used Package Manager
pnpm
Validations
- Follow our Code of Conduct
- Read the Contributing Guidelines.
- Read the docs.
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- The provided reproduction is a minimal reproducible example of the bug.
Activity
Prajwal2246 commentedon Oct 8, 2026
This looks fixed in v5. I ran the repro on a few versions:
I think #10757 fixed it: setSystemTime without fake timers doesn't use the old MockDate anymore, it uses fake-timers' clock Date, and dates created before the call are still
instanceof Date. Also passes withvi.useFakeTimers()first and aftervi.useRealTimers().So this can probably be closed as fixed in 5.0.0.