A stopwatch for measuring how long something takes, with lap times recorded as you go. Elapsed time is measured from a fixed start point rather than by counting ticks, so it stays accurate even if the display updates unevenly.
Why elapsed time is measured differently
Counting timer ticks accumulates error; comparing timestamps does not. That distinction — wall-clock time versus a monotonic elapsed measure — is the heart of millisecond precision in the browser, and it is why a stopwatch and a clock are genuinely different tools.