1. [Unix: probably 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 US 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 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