A timer is a specialized type of clock used for measuring specific time intervals.
Timers can be categorized into two main types.
A timer which counts upwards from zero for measuring elapsed time is often called a stopwatch, while a device which counts down from a specified time interval is more usually called a timer. A simple example of this type is an hourglass. Working method timers have two main groups: hardware and software timers.
Most timers give an indication that the time interval that had been set has expired.
Time switches, timing mechanisms which activate a switch, are sometimes also called "timers."