Unix time

Unix time is the number of seconds since January 1, 1970 at 00:00 UTC. Computers use it because it is a single number, without time zone or daylight saving time. A timestamp of 10 digits is in seconds, of 13 digits in milliseconds, and of 16 digits in microseconds; we recognize that automatically.

On January 19, 2038, the old 32-bit variant will overflow (the "year 2038 problem"). Modern systems use 64 bits and do not have that problem.