Online Stopwatch with Laps

Nothing is counted tick by tick. When you start, the page notes the moment on your device's clocks; every value on screen is then a subtraction: the time banked before the last pause, plus the gap between now and that start. The digits are redrawn on every frame, and that loop stops while the tab is hidden: when you come back, the first frame reads the clocks again, so no delay can build up. The display is truncated to the hundredth, never rounded up, so it never shows a hundredth that hasn't elapsed yet.

Two clocks are read together. The page's internal clock is monotonic — changing the computer's clock doesn't make it jump — but it can stop while the machine sleeps, as it does on macOS and Linux. The system clock keeps going. As long as both advance together, the monotonic clock wins; if the system clock has pulled more than two seconds ahead, the machine was asleep, and the stopwatch goes with the system clock. While paused, the banked time is frozen; “Resume” opens a new segment.

Each press of “Lap” records the total time at that instant; the lap time is the gap since the previous press, and the top row of the table follows the lap in progress. “Average lap” appears from the first lap. “Fastest lap” and “Slowest lap” wait for the second, and only if their times differ at the hundredth shown; on a tie, the earliest of the tied laps is marked. Green and red always come with a word. With a screen reader, each action is announced (“Lap 3: 00:04.50”), and a “Current time” button, which appears when you move through the page with the keyboard, reads out the elapsed time.

• Track sessions: press L every time you cross the line. For intervals, press it at the end of each rep and at the end of each recovery: odd laps are your reps, even laps your recoveries, and comparing the odd rows shows where your pace slipped.
• Swimming: at the poolside, full screen shows the time in huge digits with the last three laps, readable from a distance; press L at each wall to get the time of every length.
• Cooking, when you don't know the duration in advance: how long your caramel takes to turn amber, or a sauce to reduce. The screen stays on while it runs if your browser supports it; for a duration you already know, a countdown timer is the better fit.
• Rehearsing a talk or an oral exam: one lap per slide shows which ones drag on. In a meeting or a debate, one lap per speaker measures how long each person talks, and the export keeps the order in which people spoke.
• Timing a repetitive task — packing an order, entering an invoice, answering a ticket: one lap per item, and “Average lap” gives the average time per item; pause for breaks and they won't count.
• Physics lab: for a pendulum's period, start as it swings through the lowest point and press L each time it passes again in the same direction. “Average lap” equals the total time divided by the number of laps — only the first and last presses matter — and the gap between the fastest and slowest lap shows how much your reflexes wobble.
• Games: one lap per turn gives each player's thinking time; for a speed challenge (a puzzle, a quiz, a speedrun), Space starts and stops with a single key.

Three laps and a pause. Start, press “Lap” when the screen shows 00:05.00, pause at 00:08.00, resume 51 seconds later, then record two laps at 00:11.00 and 00:15.50. The lap times read 00:05.00, 00:06.00 and 00:04.50: the 51-second pause appears nowhere. “Fastest lap” shows 00:04.50 (lap 3), “Slowest lap” 00:06.00 (lap 2) and “Average lap” 00:05.16 — 15.5 s ÷ 3 = 5.1667 s, and the display truncates to the hundredth instead of rounding.

The same laps, exported. “Copy laps” puts four tab-separated lines on the clipboard: the header Lap, Lap time, Total time, then one row per lap — lap 3 holds 3, 0:00:04.50 and 0:00:15.50 in three columns. “Download CSV” saves stopwatch-laps.csv with commas between columns; its last line is 3,0:00:04.50,0:00:15.50. The export always writes out the hours.

Reloading. Start, record a lap at 00:10.00, then reload the page 60 seconds after the start: the display reads 01:00.00, lap 1 still shows 00:10.00 and the in-progress row for lap 2 shows 00:50.00.

Sleep. Start the stopwatch, close the laptop lid after 10 minutes and open it an hour later: at that moment the stopwatch reads 1:10:00.00, not 10:00.00, and the tab title shows “▶ 1:10:00 · Stopwatch”.

• The hundredth on screen is not the accuracy of the measurement: the delay of your finger, at the start and at the stop, weighs far more. This stopwatch is not certified timing equipment, and its times are no substitute for an official result.
• The stopwatch relies on your device's clock: moving the system clock forward by more than two seconds during a run adds that much to the measurement, and adjusting the clock between the start and a reload skews the resumed time. Daylight saving changes make no difference.
• Picking up where you left off requires the same browser, with storage allowed: if the browser refuses to save, the stopwatch still works but forgets everything on the next load, and a private window wipes the session when it closes. There is one stopwatch per browser: two tabs show the same one.
• The export only contains recorded laps: the time since the last lap isn't in it, hence pressing “Lap” before “Pause”. Times use a decimal point before the hundredths (0:00:05.00), which a spreadsheet set to a decimal-comma language may treat as text. A session holds up to 999 laps.
• Space follows focus: if a button is selected — for instance right after you clicked it — Space presses that button instead of starting or pausing. The L, R and F keys work whichever button is selected.
• Keeping the screen on depends on the browser: where the feature is missing, or once the tab goes to the background, the screen may turn off according to your device settings — the timing carries on regardless.