Most legacy aircraft came out of the factory with little round clocks in the panel, although these days they’re largely ignored in favor of the more convenient digital timers. Recall that a way of keeping time is listed among the required equipment for IFR operations—see 14 CFR 91.205(d). Technically, the “clock displaying hours, minutes, and seconds with a sweep-second pointer or digital presentation” is supposed to be certified, installed equipment, meaning it has to be the original clock or an approved replacement, and it has to be operational for legal IFR flight.
Really? Yes, really.
Keep in mind that the clock is in the same list as gyros and two-way radios, so it has the same status. While we’re mulling over that one, here’s the reality check: We likely don’t concern ourselves with that original clock in the corner of the panel, because we’ve got better ways to keep time in our flight operations. But if you want to be fully compliant, best to check that it runs. If you have digital avionics or a glass panel installed, there’s bound to be at least one digital clock that will meet the legal requirement.