NeXT-Generation:
Thanks, but that doesn't completely answer my question. As far as I can tell, the only timer example waits for the timer to overflow, not for a user-defined period. Also, the ISR would need to have access to a variable that tells it what numbers to display. Does it just need to be a global variable? Or does it need to have something special done to it?
Timers count at a specific frequency, which is user-selectable. Interrupts can occur either on overflow or when a specific value is reached, also user-selectable. Do the math on that and a pretty wide range of user-defined periods is possible.
Dean Camera and Nick Gammon both have good write-ups on timers and interrupts, and there is always the ultimate reference, i.e. the datasheet.