Epoch

The epoch (of the English time or era ) represents the initial date from which is measured the Temps by the operating systems.

Principle

This date varies according to the systems, for example:
  • under UNIX, it is January 1st 1970 at 0 a.m. (UTC)
  • under Mac OS, on January 1st 1904 at 0 a.m.
  • under Mac OS X (being based on UNIX, it uses the epoch UNIX) thus 1970 at 0 a.m.
  • under VMS, on November 17th 1858 at 0 a.m.
  • under Windows on January 1st 1900 at 0 a.m.
Time is measured of many units of time since this date. The most current unit of time is the second. It happens that the unit is not specified and that one speaks about tick , which is the smallest unit of time managed by the operating system.

Going beyond

Problems are likely to arrive when the date exceeds the maximum value authorized for the type of data used to store it. For example, under UNIX, the dates are expressed of many seconds since Epoch and are stored in entireties. When those have a size of 32 bits, problems are thus likely to occur starting from January 19th, 2038. There exists the same problem under Windows as from dated February 6th, 2036.

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