Yes, I'll do the way raschemmel suggest. Look out with:
This number will overflow (go back to zero), after approximately 50 days.
I'm not sure you if you see that as a problem, I understand it isn't. I would nonetheless suggest you use an RTC and have a regular loop with, say, a one minute delay, which checks the RTC for a specific time. The DS1307 is dirt cheap, and adequate for this.