Online Stopwatch

Why splits change how you pace

The reason serious runners and swimmers obsess over lap times is that human pacing intuition is unreliable and degrades exactly when it matters. Almost everyone starts too fast — adrenaline and fresh legs make a pace feel comfortable that is quietly unsustainable — and then pays for it in the final third. Splits make that visible while there is still time to act on it: a first lap that is eight seconds quicker than target is a warning, not an achievement. Coaches teach negative splitting, running the second half of a race slightly faster than the first, precisely because it is the pattern that produces the best times and it is the opposite of what unmonitored instinct produces. The stopwatch is not measuring your effort; it is correcting your judgement about it.

Browser timers and the throttling problem

There is a subtle bug that afflicts a great many web-based timers and it is worth understanding. The naive way to build a stopwatch is to run a repeating callback every ten milliseconds and add ten milliseconds to a counter each time. This works perfectly while the tab is in the foreground and fails badly the moment it is not, because browsers deliberately throttle background tabs to save battery — the callback may fire once a second instead of a hundred times, and the counter falls steadily behind. The timer appears to have lost minutes. The correct approach records the wall-clock time at the start and computes the elapsed time by subtraction whenever it needs to display, which is immune to throttling because it never depended on being called regularly in the first place.