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 NCSA.
