I stand to be corrected, but this is not, I believe, something that can readily achieved. You can't do this through your main OS, as the OS will be needed to run the program and so cannot be destroyed by the wipe. And it is difficult to think of any means of scheduling a separate CD based OS (such as the one on which DBAN runs) when that is not normally started with the machine.
AFAIK, no scheduler is built into DBAN, and it hasn't been supported or updated for a number of years now. I suppose you could leave a cut down CD-based Linux distro running with a scheduler which could trigger an erasing program (or a key change on an encrypted disk), but something like that would have to be built up pretty much from scratch.