I had a reminder set up that simply sent an email to people on a given date who registered for a forthcoming event using the 'pay later' option, reminding them that they need to pay. All good.
I then wanted to change the reminder so that it repeated every two weeks after that given date until two weeks after the event start date. But CiviCRM won't let me set a repeating pattern for a reminder that uses an absolute data. Or rather it does let me set up the repeating pattern, but then simply doesn't save that edit.
I can't see any good reason why I might not be able to set up a reminder that sends an email on (for example) the 1st of January, and repeats every two weeks until an event start date.
Which leads me to think that this is a bug. I've tested it on my live site (WP and Civi 4.6.21) and on http://wpmaster.demo.civicrm.org
The documentation for 4.7 (https://docs.civicrm.org/user/en/stable/email/scheduled-reminders/) makes no mention, which further makes me think this is a bug:
Choose when to send the reminder. This can be a set date or an amount of time before or after the start or end of your chosen entity. Check the Repeat box if you want to send this reminder more than once to the same group of people. This gives you choices about the interval of repetition and when the repetition should end.