iPhone alarms not working anymore

I used to wake up every morning with my iPhone 4 (iOS 5.1.1) working as an alarm clock, but with the iPhone in “airplane mode” in order not to be awaken by e-mails in the middle of the night (yes, even a vibration can wake me up…).

This worked fine for months, but since a few days it doesn’t work anymore : alarms are not fired anymore, not until I click the “home” button later. I tried it a few times, it is reproducible. After a reboot : no changes. The problem also happens in airplane mode as well as in normal mode.

I know there have been issues like this due to DST changes or new year, but has anyone had it recently ?

Thanks for any hint…