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Next: <a href="Specifying-time-zone-rules.html#Specifying-time-zone-rules" accesskey="n" rel="next">Specifying time zone rules</a>, Previous: <a href="Pure-numbers-in-date-strings.html#Pure-numbers-in-date-strings" accesskey="p" rel="prev">Pure numbers in date strings</a>, Up: <a href="Date-Input-Formats.html#Date-Input-Formats" accesskey="u" rel="up">Date Input Formats</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Function-Index.html#Function-Index" title="Index" rel="index">Index</a>]</p>
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<a name="Seconds-since-the-Epoch-1"></a>
<h3 class="section">B.8 Seconds since the Epoch</h3>
<p>If you precede a number with ‘<samp>@</samp>’, it represents an internal time
stamp as a count of seconds. The number can contain an internal
decimal point (either ‘<samp>.</samp>’ or ‘<samp>,</samp>’); 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 time stamp.
</p>
<a name="index-beginning-of-time_002c-for-POSIX"></a>
<a name="index-epoch_002c-for-POSIX"></a>
<p>Internally, computer times are represented as a count of seconds since
an epoch—a well-defined point of time. On <acronym>GNU</acronym> and
<acronym>POSIX</acronym> systems, the epoch is 1970-01-01 00:00:00 <small>UTC</small>, so
‘<samp>@0</samp>’ represents this time, ‘<samp>@1</samp>’ represents 1970-01-01
00:00:01 <small>UTC</small>, and so forth. <acronym>GNU</acronym> and most other
<acronym>POSIX</acronym>-compliant systems support such times as an extension
to <acronym>POSIX</acronym>, using negative counts, so that ‘<samp>@-1</samp>’
represents 1969-12-31 23:59:59 <small>UTC</small>.
</p>
<p>Traditional Unix 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 <small>UTC</small>. More modern systems use 64-bit counts
of seconds with nanosecond subcounts, and can represent all the times
in the known lifetime of the universe to a resolution of 1 nanosecond.
</p>
<p>On most hosts, these counts ignore the presence of leap seconds.
For example, on most hosts ‘<samp>@915148799</samp>’ represents 1998-12-31
23:59:59 <small>UTC</small>, ‘<samp>@915148800</samp>’ represents 1999-01-01 00:00:00
<small>UTC</small>, and there is no way to represent the intervening leap second
1998-12-31 23:59:60 <small>UTC</small>.
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