How can I schedule downloads to off peak hours?

My internet plan has 20GB during the day and 20GB off peak available from 2am to 10am. How can I schedule large downloads like IOS7 to occur during the off peak hours? Is there a download manager that will help? The only apps I have found don't seem to offer scheduled start times. Could I perhaps use something like Apple Script to write a routine to do this? maybe using calendar events? I will probably also want to do the same thing on my iMac someday. Any help much appreciated,

Cheers,

DaveD.