Stopwatches and timers are instruments used to measure time interval, which
is defined as the elapsed time between two events. One common example
of a time interval is a person’s age, which is simply the elapsed time since
the person’s birth. Unlike a conventional clock that displays time-of-day as
hours, minutes, and seconds from an absolute epoch or starting point (such as
the beginning of the day or year), a stopwatch or timer simply measures and
displays the time interval from an arbitrary starting point that begins at the
instant when the stopwatch is started.