An alarm clock is an item that displays the time.
When it is in the player's main inventory, the time of day will be displayed on the HUD at the top right of the screen. An alarm can be set to go off at a specific time by right-clicking it and selecting "Set Alarm". The alarm can be turned on or off.
This is a very effective tool for drawing off zombie hordes. Once the alarm is triggered (lasts for around 10 seconds), zombies move towards it and stop on top of it and disperse once it ends. This device will not prevent zombies from seeing the player, however can be used to lure them to a certain area.
The sound of a triggered alarm will wake up the player if they're sleeping. But be careful, this will also draw nearby zombies.
Alarm clocks can be found in house bedrooms.
From newitems.txt (Project Zomboid directory/media/scripts/)
Type = AlarmClock,
DisplayName = Alarm Clock,
Weight = 0.5,
Icon = AlarmClock,
AlarmSound = AlarmClockRingingLoop,
SoundRadius = 15,
MetalValue = 25,