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.
![[prev]](../gif/left.gif){kind=small}
![[up]](../gif/up.gif){kind=small}
![[next]](../gif/right.gif){kind=small}