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.

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 UTC. 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.

1972-09-24 # ISO 8601.

  -R, --rfc-2822  
         output date and time in RFC 2822 format.  Example: Mon,   07 Aug 2006 12:34:56 -0600

  -u, --utc, --universal  
         print or set Coordinated Universal Time