13 Facts About Unix time

Unix time is a system for describing a point in time.

Unix time is not a true representation of UTC, because a leap second and the second before it have the same Unix time.

Unix time originally appeared as the system time of Unix, but is used widely in computing, for example by filesystems; some Python language library functions handle Unix time.

Unix time is a single signed number that increments every second, which makes it easier for computers to store and manipulate than conventional date systems.

Unix time number is zero at the Unix epoch and increases by exactly 86 per day since the epoch.

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Sometimes, Unix time is mistakenly referred to as Epoch time, because Unix time is based on an epoch and because of a common misunderstanding that the Unix epoch is the only epoch.

Unix time numbers are repeated in the second immediately following a positive leap second.

When dealing with periods that do not encompass a UTC leap second, the difference between two Unix time numbers is equal to the duration in seconds of the period between the corresponding points in time.

Unix time number is easily converted back into a UTC time by taking the quotient and modulus of the Unix time number, modulo 86.

Unix time is not a suitable way to represent times prior to 1972 in applications requiring sub-second precision; such applications must, at least, define which form of UT or GMT they use.

However, there was no consideration of the details of Unix time scales, and it was implicitly assumed that there was a simple linear Unix time scale already available and agreed upon.

Several later problems, including the complexity of the present definition, result from Unix time having been defined gradually by usage rather than fully defined from the outset.

The POSIX committee considered whether Unix time should remain, as intended, a linear count of seconds since the epoch, at the expense of complexity in conversions with civil time or a representation of civil time, at the expense of inconsistency around leap seconds.