3 days ago

GNU: date --date '3 days ago'

BSD: date -v -3d

From UNIX epoch

GNU: date --date @1594956336

BSD: date -r 1594956336

So what GNU date can do with --date? What is the syntax?

From man date(1);

The --date=STRING is a mostly free format human readable date string such as “Sun, 29 Feb 2004 16:21:42 -0800” or “2004-02-29 16:21:42” or even “next Thursday”. A date string may contain items indicating calendar date, time of day, time zone, day of week, relative time, relative date, and numbers. An empty string indicates the beginning of the day. The date string format is more complex than is easily documented here but is fully described in the info documentation.

Oh, yeah. I love this stubborn GNU-ism! The quote in the info documentation is also fascinating.