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, the same as a digital watch and classic wrist watch, with additional weight. Similar to the classic wrist watch, the alarm clock will not display how long the player has been alive for in the info tab. 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.