http://en.wikipedia.org/wiki/Unix_timeEncoding time as a number
Unix time is a single signed integer number which increments every second, without requiring the calculations to determine year, month, day of month, hour and minute required for intelligibility to humans. Modern Unix time is based strictly on UTC, which counts time using SI seconds, and breaks up the span of time into days almost always 86 400 s long, but due to leap seconds occasionally 86 401 s and could be 86 399 s long (though the latter option had never been used as of December 2010); this keeps the days synchronized with the rotation of the Earth (or Universal Time). As is standard with UTC, this article labels days using the Gregorian calendar, and counts times within each day in hours, minutes, and seconds. Some of the examples also show TAI, another time scheme, which uses the same seconds and is displayed in the same format as UTC, but in which every day is exactly 86 400 s long, gradually losing synchronization with the Earth's rotation at a rate of roughly one second per year.
The Unix epoch is the time 00:00:00 UTC on 1 January 1970 (or 1970-01-01T00:00:00Z ISO 8601). There is a problem with this definition, in that UTC did not exist in its current form until 1972; this issue is discussed below. For brevity, the remainder of this section uses ISO 8601 date format, in which the Unix epoch is 1970-01-01T00:00:00Z.
The Unix time number is zero at the Unix epoch, and increases by exactly 86 400 per day since the epoch. Thus 2004-09-16T00:00:00Z, 12 677 days after the epoch, is represented by the Unix time number 12 677 × 86 400 = 1 095 292 800. This can be extended backwards from the epoch too, using negative numbers; thus 1957-10-04T00:00:00Z, 4 472 days before the epoch, is represented by the Unix time number -4 472 × 86 400 = -386 380 800.Within each day, the Unix time number is as calculated in the preceding paragraph at midnight UTC (00:00:00Z), and increases by exactly 1 per second since midnight. Thus 2004-09-16T17:55:43.54Z, 64 543.54 s since midnight on the day in the example above, is represented by the Unix time number 1 095 292 800 + 64 543.54 = 1 095 357 343.54. On dates before the epoch the number still increases, thus becoming less negative, as time moves forward.
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