[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 {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}. 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.
[glossary]
[Reference(s) to this entry by made by: {era}{since time T equals minus infinity}]