The Stopwatch tab allows you to use your phone as a Stopwatch.
Open the Clock app, then select the Stopwatch tab (if necessary).
Tap the green Start button (at the bottom of the screen) to start timing and tap the red Stop button to stop the time.
You can use the Lap function by tapping the Lap button to record multiple milestone times within the same timing session.
Tap the Reset button to reset the Stopwatch or tap the green Resume button (at the bottom of the screen) to continue monitoring time.