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UNIX time is seconds since epoch (hence year 2038, that's the limit for a signed 32b time_t).

gettimeofday() and clock_gettime() provide higher resolution timestamps (respectively µs and ns), using typedefs instead of just numbers.

Some APIs return floating-point UNIX time in order to provide sub-second accuracy (the decimal part is the fractional second). Python's time.time() does that for instance.

It’s a signed type so dates before 1970 can be represented

time_t is usually signed.