A timer or countdown timer is a type of clock that starts from a specified time duration and stops when reaching 00:00. A simple timer is an hourglass. Commonly, a timer triggers an alarm when it ends. A timer can be implemented through hardware or software. Stopwatches operate in the opposite direction, upwards from 00:00, measuring elapsed time since a given time instant. Time switches are timers that control an electric switch.

A stopwatch is a timepiece designed to measure the amount of time that elapses between its activation and deactivation.