POSIX time (Unix epoch time)

Unix time passed 1,234,567,890 seconds February 13, 2009, at exactly 23:31:30 (UTC).

Unix time, or POSIX time, is a system for describing points in time, defined as the number of seconds elapsed since midnight Coordinated Universal Time (UTC) of January 1, 1970, not counting leap seconds. It is widely used not only on Unix-like operating systems but also in many other computing systems. It is neither a linear representation of time nor a true representation of UTC (though it is frequently mistaken for both) as the times it represents are UTC but it has no way of representing UTC leap seconds (e.g. 1998-12-31 23:59:60).
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The Unix epoch is the time 00:00:00 UTC on January 1, 1970
There is a problem with this definition, in that UTC did not exist in its current form until 1972; this issue is discussed below. For brevity, the remainder of this section uses ISO 8601 date format, in which the Unix epoch is 1970-01-01T00:00:00Z.