`date instanceof Date` returns false for dates created before `vi.setSystemTime()`

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

https://github.com/diego-aquino/vitest-set-system-time-instanceof

1. pnpm install
2. pnpm 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.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion or join our Discord Chat Server.
• The provided reproduction is a minimal reproducible example of the bug.

Run the linked reproduction with pnpm install and pnpm test, starting from example.test.ts. Trace the Date mocking behavior around vi.setSystemTime; done means the pre-existing Date passes toBeInstanceOf(Date) after system time changes while the reproduction test remains passing.