How to set Cron Job in custom module for specific time in Drupal 7
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).


Important: You cannot assume your task will be executed at specific time. It will be executed at first opportunity after the time you specified, whenever, if ever, such opportunity happens. Elysia Cron is pretty good at it, true, but exact time of execution can't be assumed guaranteed. Highly probable, to some resolution, maybe. But never guaranteed.