epoch n.
[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 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.