Epoch converter

Negative epoch values are perfectly valid and represent dates before 1970. Epoch −86400 is 31 December 1969. A surprising amount of software mishandles them, either rejecting the value or producing a date decades out, because the implementation assumed unsigned arithmetic. It is worth testing if you are handling historical dates rather than log timestamps. The other thing epoch time is not is a count of elapsed seconds. Every day in Unix time is defined as exactly 86,400 seconds, so when a leap second is inserted the clock repeats a value rather than counting past it. Twenty-seven leap seconds have been added since 1972, which means the real interval between two epoch values that far apart is 27 seconds longer than the subtraction says. That is deliberate; it keeps epoch arithmetic to plain division and lets every date convert without a lookup table — and it is why epoch time is not the same thing as atomic time. International agreement in 2022 committed to abandoning leap seconds by 2035, at which point the discrepancy stops growing and simply stays where it is.