Timer is a fairly simple timer with two modes, manual and automatic.

In manual mode, left-clicking will start and stop Timer. This is designed as a general timer, for those curious about how long a keep took to take, how long they've been logged in and so on.

In automatic mode it will start on entering combat and stop on exiting combat. This is designed as a timer for dungeon bosses, so you can keep an eye on how long you have until the boss enrages, in particular appropriate for Tomb of the Vulture Lord bosses, with the many DPS races involved.

To change modes, clear the timer or check the previous timer value, simply right-click the window and select your option.