I have an iphone4 that hasn't been jailbroken, I have had this problem for 2 days now (in UK) tried deleting and reprogramming alarms with no success, but then read the above post regarding his girlfriends phone clock not being set to automatically update.
Changed mine to manual and it seems to work now. I set the alarm to go off at the correct time a few minutes ahead and it went off as expected.
In short, switch auto time set off and redo your alarms. Seems to work. (i'll let you know tomorrow!)😛