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10. Time > Calendar Time

Calendar Time

Regardless of geographic location, UNIX systems represent time internally as a measure of seconds since the Epoch; that is, since midnight on the morning of 1 January 1970, Universal Coordinated Time (UTC, previously known as Greenwich Mean Time, or GMT). This is approximately the date when the UNIX system came into being. Calendar time is stored in variables of type time_t, an integer type specified by SUSv3.

Note

On 32-bit Linux systems, time_t, which is a signed integer, can represent dates in the range 13 December 1901 20:45:52 to 19 January 2038 03:14:07. (SUSv3 leaves the meaning of negative time_t values unspecified.) Thus, many current 32-bit UNIX systems face a theoretical Year 2038 problem, which they may encounter before 2038, if they do calculations based on dates in the future. This problem will be significantly alleviated by the fact that by 2038, probably all UNIX systems will have long become 64-bit and beyond. However, 32-bit embedded systems, which typically have a much longer lifespan than desktop hardware, may still be afflicted by the problem. Furthermore, the problem will remain for any legacy data and applications that maintain time in a 32-bit time_t format.


  

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