You can't. Drupal 7 does not provide this functionality. You have two options:
In your
mymodule_cron(), test if it is after specified time, and at least arbitrary number of hours after previous run (like, 20?). That way your processing will run almost every day, no earlier than specified hour.Make your module require Elysia Cron and use it's scheduling capabilities. That way you will get your code executed everyday, at specified time (5 minutes resolution if Elysia is configured as suggested in their documentation).