GNU Mailutils 

GeneralPurpose Mail Package 
Official GNU Software 
If you precede a number with ‘@’, it represents an internal time stamp 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 time stamp.
Internally, computer times are represented as a count of seconds since an epoch—a welldefined point of time. On GNU and POSIX systems, the epoch is 19700101 00:00:00 UTC, so ‘@0’ represents this time, ‘@1’ represents 19700101 00:00:01 UTC, and so forth. GNU and most other POSIXcompliant systems support such times as an extension to POSIX, using negative counts, so that ‘@1’ represents 19691231 23:59:59 UTC.
Traditional Unix systems count seconds with 32bit two’scomplement integers and can represent times from 19011213 20:45:52 through 20380119 03:14:07 UTC. More modern systems use 64bit 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.
On most hosts, these counts ignore the presence of leap seconds. For example, on most hosts ‘@915148799’ represents 19981231 23:59:59 UTC, ‘@915148800’ represents 19990101 00:00:00 UTC, and there is no way to represent the intervening leap second 19981231 23:59:60 UTC.
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