1. <operating system> (Probably from astronomical timekeeping) A term
used originally in Unix documentation for the time and date corresponding to
zero in an operating system's clock and timestamp values.
Under most Unix versions the epoch is 1970-01-01 00:00:00 GMT; under VMS, it's
1858-11-17 00:00:00 (the base date of the US Naval Observatory's ephemerides);
on a Macintosh, it's 1904-01-01 00:00:00.
System time is measured in seconds or ticks 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 0.1 * 2**31-1 seconds, or 6.8 years. The
one-tick-per-second clock of Unix is good only until 2038-01-18, assuming at
least some software continues to consider it signed and that word lengths don't
increase by then. See also wall time.
2. <editor> (Epoch) A version of GNU Emacs for the X Window System from
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