epoch

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

wall time.

2. (Epoch) A version of GNU Emacs for the X Window System

from NCSA.

[Jargon File]