A long, long time ago... there was a Unix (not necessarily Linux[1]) program that would spit out today's date in a variety of formats.
In other words, it did what http://www.public.asu.edu/~checkma/today.html does, but in pure text, not HTML (HTML didn't exist at the time), and with fewer outputs.
The program I'm thinking of isn't gcal (gcal didn't exist at the time, and GNU wasn't well known), but some of the gcal manuals and online tutorials suggest gcal can also do this. However, I haven't been able to figure out how.
[1] It was probably SunOS; I think it was before SOLARIS came out.
EDIT: I just found a program called 'ddate' that does a very small part of this by returning the Discordian date (but not the others):
> ddate
Today is Pungenday, the 45th day of Discord in the YOLD 3183
It turns out calendar -f /usr/share/calendar/calendar.discord -A 0
will spit out the same output (which surprised me, since I'd forgotten that 'calendar' was more than a reminder program), but is also very limited in choice of calendars.
A program called hebcal (https://www.gsp.com/cgi-bin/man.cgi?section=1&topic=hebcal) apparently did this for Hebrew calendars, but I'm sure what I saw listed the same date in multiple formats-- it's possible it combined hebcal, ddate, and other programs in a shell script.