A timer is a type of agent that creates an invisible timing object able to count real time accurately and dispatch synchronous events. They are capable of triggering events or executing functions on after a certain amount of time (expiration) or periodically. A timeout of 0 does have a delay.
Trigger Editor
Unlike many other objects, you don't need to initialize a timer. There is no Countdown Timer - Create Timer, but you do need to have a variable of type Timer.
Precision
Timing is accurate to ??? milliseconds. However, there is another limitation due to how the Variable Editor in the World Editor limits real values to 3 decimal places.
Timers in JASS
A timer does in fact have to be created using the function CreateTimer(), which returns a timer.