Unix time

#Calendaring_standards
#Network_time-related_software
#Time_measurement_systems
#Time_scales
#Unix
#1970
Current Unix time 1642101106 （2022-01-13T19:11:46+00:00） Unix time passed 1000000000 seconds on 2001-09-09T01:46:40Z. It was celebrated in Copenhagen, Denmark at a party held by DKUUG （at 03:46:40 local time）.
Unix time （also known as Epoch time, Posix time, seconds since the Epoch, or UNIX Epoch time） is a system for describing a point in time.
It is the number of seconds that have elapsed since the Unix epoch, excluding leap seconds.
The Unix epoch is 00:00:00 UTC on 1 January 1970 （an arbitrary date）.
Unix time is nonlinear with a leap second having the same Unix time as the second before it （or after it, implementation dependent）, so that every day is treated as if it contains exactly 86400 seconds,
with no seconds added to or subtracted from the day as a result of positive or negative leap seconds.
Due to this treatment of leap seconds, Unix time is not a true representation of UTC. Unix time is widely used in operating systems and file formats.
In Unix-like operating systems, date is a command which will print or set the current time; by default, it prints or sets the time in the system time zone, but with the -u flag,
it prints or sets the time in UTC and, with the TZ environment variable set to refer to a particular time zone, prints or sets the time in that time zone.
Two layers of encoding make up Unix time.
The first layer encodes a point in time as a scalar real number which represents the number of seconds that have passed since 00:00:00 UTC Thursday, 1 January 1970.
The second layer 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...