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Many computer systems measure time and date using Unix time, an international standard for digital timekeeping.Unix time is defined as the number of seconds elapsed since 00:00:00 UTC on 1 January 1970 (an arbitrarily chosen time based on the creation of the first Unix system), which has been dubbed the Unix epoch.
Between these adjustments, it is composed of segments that are mapped to atomic time by a constant offset. From its beginning in 1961 through December 1971, the adjustments were made regularly in fractional leap seconds so that UTC approximated UT2. Afterwards, these adjustments were made only in whole seconds to approximate UT1. This was a ...
It has since been set backward 8 times and forward 17 times. The farthest time from midnight was 17 minutes in 1991, and the nearest is 90 seconds, set in January 2023. The Clock was moved to 150 seconds (2 minutes, 30 seconds) in 2017, then forward to 2 minutes to midnight in January 2018, and left unchanged in 2019. [6]
10:07: NBC reports for the first time that the South Tower of the World Trade Center has collapsed. Prior to this time they have said only that a section of the building has fallen away. 10:07: NEADS, controlling the only set of fighters over Washington, first learns of the hijacking of Flight 93, 4 minutes after it actually crashed.
For example, the Unix system time 1 000 000 000 seconds since the beginning of the epoch translates into the calendar time 9 September 2001 01:46:40 UT. Library subroutines that handle such conversions may also deal with adjustments for time zones , daylight saving time (DST), leap seconds, and the user's locale settings.
Unix time numbers are repeated in the second immediately following a positive leap second. The Unix time number 1 483 228 800 is thus ambiguous: it can refer either to start of the leap second (2016-12-31 23:59:60) or the end of it, one second later (2017-01-01 00:00:00). In the theoretical case when a negative leap second occurs, no ambiguity ...
On clocks that display local time tied to UTC, the leap second may be inserted at the end of some other hour (or half-hour or quarter-hour), depending on the local time zone. A negative leap second would suppress second 23:59:59 of the last day of a chosen month so that second 23:59:58 of that date would be followed immediately by second 00:00: ...
The exact moment is dependent on the geographical longitude, and to a lesser extent on the time of the year. The length of such a day is nearly constant (24 hours ± 30 seconds). This is the time as indicated by modern sundials.