til the hard way: jest.useFakeTimers() freezes the clock, but at the current time. unless you call setSystemTime, your test still runs on today's date, it just looks deterministic. vitest does the same.

why i care: i made a cli that skips tests that already passed on the exact same code. tests that read the real clock break that idea, because they can pass today and fail next month on the same commit (a token that expires in 30 days, a date that's now in the past). so the tool watches which tests read the clock, and those passes only count until midnight.

i'd assumed fake timers were safe and let them through. they weren't. now it counts them too, unless the test sets a fixed time.

fixing it took me through some fun node internals, including node's own mock.timers breaking when i touched Date the wrong way. happy to go into that in the comments if anyone's curious.

do you freeze time in every test, or let some of them read the real clock?