Start up your 64DD with no disk inserted and hold down A(or B, i forgot) for a few seconds and a screen will pop up where you can adjust the time and date.
I am looking for a 64DD Keyboard, Japan Pro Golf Tour, Doshin 2, and then my DD collection will be complete. I am also looking for the Millennium controller. Currently hooked on Bomberman 64 and Banjo-Tooie =)