[Unix: prob. from astronomical timekeeping] 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 ticks past the epoch. Weird
problems may ensue when the clock wraps around (see wrap around), which is not necessaril
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. Microsoft Windows, on
the other hand, has an epoch problem every 49.7 days - but this
is seldom noticed as Windows is almost incapable of staying
up continuously for that long.
