epoch: [UNIX: prob. from astronomical timekeeping] n.

:epoch: [UNIX: prob. from astronomical timekeeping] n. The time
and date corresponding to 0 in an operating system's clock and
timestamp values. Under most UNIX versions the epoch is 00:00:00
GMT, January 1, 1970; under VMS, it's 00:00:00 of November 17, 1858
(base date of the U.S. Naval Observatory's ephemerides); on a
Macintosh, it's the midnight beginning January 1 1904. System time
is measured in seconds or {tick}s past the epoch. Weird
problems may ensue when the clock wraps around (see {wrap
around}), which is not necessarily a rare event; on systems
counting 10 ticks per second, a signed 32-bit count of ticks is
good only for 6.8 years. The 1-tick-per-second clock of UNIX is
good only until January 18, 2038, assuming at least some software
continues to consider it signed and that word lengths don't
increase by then. See also {wall time}.
-- The AI Hackers Dictionary