File: coreutils.info,  Node: Seconds since the Epoch,  Next: Specifying time zone rules,  Prev: Pure numbers in date strings,  Up: Date input formats

29.9 Seconds since the Epoch
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If you precede a number with ‘@’, it represents an internal timestamp as
a count of seconds.  The number can contain an internal decimal point
(either ‘.’ or ‘,’); any excess precision not supported by the internal
representation is truncated toward minus infinity.  Such a number cannot
be combined with any other date item, as it specifies a complete
timestamp.

   Internally, computer times are represented as a count of seconds
since an Epoch—a well-defined point of time.  On GNU and POSIX systems,
the Epoch is 1970-01-01 00:00:00 UTC, so ‘@0’ represents this time, ‘@1’
represents 1970-01-01 00:00:01 UTC, and so forth.  GNU and most other
POSIX-compliant systems support such times as an extension to POSIX,
using negative counts, so that ‘@-1’ represents 1969-12-31 23:59:59 UTC.

   Most modern systems count seconds with 64-bit two’s-complement
integers of seconds with nanosecond subcounts, which is a range that
includes the known lifetime of the universe with nanosecond resolution.
Some obsolescent systems count seconds with 32-bit two’s-complement
integers and can represent times from 1901-12-13 20:45:52 through
2038-01-19 03:14:07 UTC.  A few systems sport other time ranges.

   On most hosts, these counts ignore the presence of leap seconds.  For
example, on most hosts ‘@1483228799’ represents 2016-12-31 23:59:59 UTC,
‘@1483228800’ represents 2017-01-01 00:00:00 UTC, and there is no way to
represent the intervening leap second 2016-12-31 23:59:60 UTC.