The way it works is you have your time in the PARAMS thing on your description.ext, I assume you know how to do this because I don't remember how
Anyway, in the mission you set the time to 00:00. And in your init.sqs you put:
skipTime params1
Assuming that params1 is your "Select time" thing. So in the value-field of your params you write the number of hours to skip ahead from 00:00, and et voilá you've got changable time.