ddate was written by Dru'el the Chaotic, and was later modified by Pope Lee H:. O:. Smith, KYTP, to be more Unixy.
If you're likewise a Unixy kind of person, click on the link below to get a copy of Dru'el's original C code, L:. H:. O:.'s modified code and the man page to the latter-mentioned version.
If you're not Unixy, you can grab the tar/gzip'd file, figure out how to untar/ungzip it, and see if it will compile on your platform. For this, I make no guarantees.
Snag a copy of ddate.tar.gz