If you’ve scheduled the Do Not Disturb feature on your iPhone or iPad running iOS 6, you may have noticed you don’t receive calls and alerts when you’re supposed to.
The bad news is that the only thing you can do now to solve the problem is manually turn off and on the Do Not Disturb feature. To do this, go to Settings > Notifications > Do Not Disturb and switch Scheduled to Off.
The good news is that this bug will fix itself after January 7, 2013 and you’ll be able to schedule Do Not Disturb times without any problems at all.
Apple Support doesn’t explain where this bug comes from or why it’ll be fixed on January 7th, but it’s good to know that it won’t be long before it goes away.
Source [Apple Support]