getMockedSystemTime() returns null if the fake time is specified using useFakeTimer({now: fakeTime})

[xuhdev]

Describe the bug

getMockedSystemTime() returns null if the fake time is specified using useFakeTimer({now: fakeTime}), while it correctly returns the mocked system time if using setSystemTime().

Reproduction

Run vitest with the following test:

import {vi, test, expect} from "vitest";

test("Speicify with now", () => {
    vi.useFakeTimers({
        now: 333
    });
    expect(vi.getMockedSystemTime()).toBeNull();  // <---- Shouldn't be null
})

test("Speicify with setSystemTime", () => {
    vi.useFakeTimers();
    vi.setSystemTime(333);
    expect(vi.getMockedSystemTime()?.getTime()).toBe(333);
})

The test will pass, but the first test shouldn't pass.

System Info

System:
    OS: Linux 6.12 Fedora Linux 41 (Workstation Edition)
    CPU: (12) x64 AMD Ryzen 5 8400F 6-Core Processor
    Memory: 56.15 GB / 62.40 GB
    Container: Yes
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 22.13.0 - ~/.nvm/versions/node/v22.13.0/bin/node
    Yarn: 1.22.22 - ~/.nvm/versions/node/v22.13.0/bin/yarn
    npm: 11.0.0 - ~/.nvm/versions/node/v22.13.0/bin/npm
    pnpm: 9.14.2 - ~/.nvm/versions/node/v22.13.0/bin/pnpm
  npmPackages:
    vitest: ^3.0.3 => 3.0.3

Used Package Manager

npm

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[hi-ogawa]

I created a stackblitz repro https://stackblitz.com/edit/vitest-dev-vitest-z1nfegqo?file=test%2Fbasic.test.ts

Though documentation explicitly says setSystemTime, I think it makes sense to consider { now: ... } options too. https://vitest.dev/api/vi.html#vi-getmockedsystemtime

Returns mocked current date that was set using setSystemTime.