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Unix time
Unix time (also known as POSIX time or Epoch time) is a system for describing instants in time, defined as the number of seconds that have elapsed since 00:00:00 Coordinated Universal Time (UTC), Thursday, 1 January 1970,〔(【引用サイトリンク】title=The Open Group Base Specifications Issue 7, section 4.15 Seconds Since the Epoch )〕〔Unix times are defined, but are negative, before 1 January 1970.〕 not counting leap seconds.〔〔(【引用サイトリンク】title=The Open Group Base Specifications Issue 7, Rationale, section 4.15 Seconds Since the Epoch )〕〔Leap seconds are scheduled by the International Earth Rotation and Reference Systems Service and are not predictable.〕 It is used widely in Unix-like and many other operating systems and file formats. Because it does not handle leap seconds, it is neither a linear representation of time nor a true representation of UTC.〔Unix time cannot unambiguously represent UTC leap seconds (e.g. 31 December 1998 23:59:60), although otherwise the times it represents are UTC.〕 Unix time may be checked on most Unix systems by typing on the command line.
==Definition==
Two layers of encoding make up Unix time. These can usefully be separated. The first layer encodes a point in time as a scalar real number, and the second encodes that number as a sequence of bits or decimal digits.
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 International Atomic Time (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 86400 seconds long, gradually losing synchronization with the Earth's rotation at a rate of roughly one second per year.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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