If you are in the US and have set a recurring alarm on your iPhone or iPod Touch then please note that the alarm will go off an hour late due to switch back to Standard time from Daylight Saving Time tonight at 2:00 am.
In the support document, Apple has described the symptoms as follows:
“In some regions, shortly before or after the daylight saving time (DST) change, repeating alarms created in the Clock app may work incorrectly”
Apple has provided the following solution to resolve the issue until iOS 4.2 is released:
To resolve this behavior for existing alarms, set the repeat interval to Never. You will need to reset these alarms for each day you need them.
After the switch back to Standard Time from DST tonight, you can set your alarms to repeat again.
Please don’t forget to tell your family and friends who own an iPhone or iPod Touch about the bug and how to resolve it.