Stopwatch

An online stopwatch measures elapsed time from when you press Start until you stop it, with lap splits in between. This stopwatch is built on two modern browser timing primitives. The display refreshes with requestAnimationFrame, which paints in sync with the screen at roughly 60 frames per second, so the milliseconds glide smoothly rather than jumping. The actual time math uses performance.now(), a high-resolution clock that is monotonic, meaning it only moves forward and is unaffected by system clock changes, time zone shifts, or daylight saving. That distinction matters: a naive stopwatch that counts setInterval ticks drifts and can even run backwards if the user adjusts the system clock, whereas measuring the delta from a fixed start timestamp stays accurate. Press Lap while running to capture a split; laps stack newest-first so your most recent split sits at the top, which is handy on a phone. Stop pauses the count and Start resumes from the same accumulated time without losing any milliseconds, and Reset clears both the time and the lap list. A Copy All button serializes every lap as plain text you can paste into a note, spreadsheet, or workout log. There is one honest caveat about background tabs: browsers throttle inactive tabs, so the on-screen number can freeze while the tab is hidden, but because the elapsed time is recomputed from the start timestamp the instant you return, the value shown is still correct. Nothing is uploaded, synced, or logged; the stopwatch state lives only in your tab and disappears when you close it.