Alarm Clock is a feature allowing a mobile phone to act like a common alarm clock but with more flexibility.
Currently, all mobile phones offer this functionality with varying feature sets. For example, most devices allow you to set an unlimited number of alarms and set them to repeat on a daily or weekly basis. Some even allow a customizable snooze period and with the most advanced devices you can silence an alarm just by turning the device face down.
Using a favorite FM radio station as an alarm tone is also an option with some handsets.
Some phones require the device to be on for the alarm to work while others do not.