A stopwatch measures elapsed time — start, stop, read the difference.
Drag the hands
12:00
24-hour: 00:00
12 o'clock
It's all subtraction
A stopwatch measures elapsed time — the gap between a start and a stop. Note the start, note the stop, take the difference. The hard part in practice is reading fractions of a second, which is why electronic timers replaced thumbs on buttons.
Try it
A race starts at 10:14:32 and ends at 10:17:08. How long was it?
From 10:14:32 to 10:17:00 is 2 min 28 s; plus 8 s more = 2 min 36 s (156 seconds).
When elapsed time crosses a minute or hour mark, borrow like in subtraction: 1 minute = 60 seconds, so 3 min 05 s − 1 min 40 s = 2 min 65 s − 1 min 40 s = 1 min 25 s.