Support timestamp in the format @%s (seconds since epoch) like GNU date

[nabellows]

A lot of my utilities handle dates in the seconds-since-epoch format, and I cannot portably use GNU date on MacOS to parse these back to another format for fd without slow means such as python wrapper. I think it is a standard enough date format and not ambiguous when prefixed with '@' just as GNU date can handle via date -d.

[tmccombs]

This was added by #1494