shtool-mdate(1) on Linux
The text 990dddd68f3b, as Debian 4.0 (Etch) shipped it.
GNU shtool pretty-print last modification time
Flags it lists
-
--digits -
--field-sep -
--newline -
--order -
--shorten -
--zero -
-d -
-f -
-n -
-o -
-s -
-z