Well Anopes callbacks are not really cron like, they get set off in x seconds, as the core loop goes around.. so to figure out how to execute at a given time you gotta do some math on the modules load
1. Find the current time let say that is 14:00
2. Take the current time and find the start points of the call backs
8:00 -> 18 hours
20:00 -> 6 hours
3. Set the first call back timer to be for 6 hours and 18 hours
4. In the call back function themselves set a 24 hour call back so that they are on a regular time after that, if they were evenly spaced then you could use one function
Note that since anope's main loop may not reach back to the top quickly or may idle, that any timed event will execute when the main loop is completed.