Print the date, in the format
The options are
Tue Aug 16 17:03:52 CDT 1977
−u Report Greenwich Mean Time (GMT) rather than local time.
−n Report the date as the number of seconds since the epoch, 00:00:00
GMT, January 1, 1970.
The conversion from Greenwich Mean Time to local time depends
on the $timezone environment variable; see ctime(3).
If the optional argument seconds is present, it is used as the
time to convert rather than the real time.