epoch

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 000000
   GMT, January 1, 1970; under VMS, it's 000000 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}.