> If you're on a POSIX machine.

Don't you just mean some Linux distros & BSDs? I don't see any indication that ncal is standardized by POSIX: http://pubs.opengroup.org/onlinepubs/9699919799/idx/utilitie...

According to the Linux manpage it's a FreeBSD command originally introduced in FreeBSD 2.2.6 in 1998: https://www.freebsd.org/releases/2.2.6R/announce.html