Stopwatch, Countdown Timer & Pomodoro

About Stopwatch, Countdown Timer & Pomodoro

This page combines three ways of measuring time. The Stopwatch counts up from zero and records laps. The Timer counts down either a set duration (up to 999:59:59) or the time left until a specific date and time. The Pomodoro mode cycles through focus and break phases — 25 minutes of focus, 5-minute short breaks and a 15-minute long break after every fourth focus block by default — and keeps a count of the focus sessions you finish.

All three share one design decision that matters more than it looks. None of them adds or subtracts a fixed amount on every tick. Each one stores a timestamp — when the stopwatch started, or when the countdown is due to end — and recalculates "now minus start" every time it redraws. That is the difference between a timer that is still correct when you come back from another tab and one that has quietly fallen minutes behind.

Why browser timers drift in background tabs

Browsers deliberately slow down JavaScript timers in tabs you cannot see. Since Chrome 88, timers in a hidden page are batched and checked roughly once per second. If the page has been hidden for more than five minutes, has been silent for more than 30 seconds and is running a chain of repeating timers, Chrome moves it to "intensive throttling" and checks those timers only once per minute. It is a battery-saving policy, and a web page cannot opt out of it.

A timer written as "every 250 ms, subtract 250 ms" breaks badly under that policy: while it only runs once a minute, a full minute passes in reality but the display loses a quarter of a second. That is how a 25-minute Pomodoro in a background tab can end up taking far longer than 25 minutes.

This tool avoids that by anchoring everything to a fixed start or end time and recomputing from it. When you switch back, the very next redraw shows the true elapsed or remaining time. That reference is the browser's monotonic clock (performance.now()), not the system clock, so changing the computer's time or an automatic sync correction while it runs does not make the reading jump. The stopwatch, timer and Pomodoro all save their progress to browser storage, so a page reload picks up where it left off, still running.

What can still arrive late: the alarm and the phase switch

Correct arithmetic does not help if nobody is awake to notice the deadline. The check that says "time is up" also runs inside a timer, so in a tab that has been hidden for a while it can fire up to about a minute late, and the beep plays at that moment rather than at zero.

In Pomodoro mode with auto-start switched on, the phase-change chime can be late by the same amount. The schedule itself does not slip: each new phase is timed from when the previous one was due to end, not from when the end was noticed, and when you return to the tab every phase that ended in the meantime is caught up at once. If the sound has to be on time — a speaking slot, a cooking step — keep the tab visible, for example in its own small window. Visible pages are not throttled.

Stopwatch: what the hundredths digit really tells you

The display refreshes about every 33 ms. Under one hour it reads MM:SS.CC, above that H:MM:SS.CC. The last two digits are hundredths of a second, truncated rather than rounded.

Display resolution is not the same as accuracy. With a hand-operated stopwatch the dominant error is the person pressing the button: human visual reaction time is in the region of a fifth of a second, and you pay it twice, once at start and once at stop, though the two partly cancel. Treat the hundredths as a readout, not as proof of a 0.01-second difference between two runners. It is the same reason athletics distinguishes hand timing from fully automatic timing.

Once you have at least two laps, the fastest and slowest are highlighted. If every lap is identical, nothing is highlighted.

Timer: a duration or a moment on the calendar

In duration mode the tool fixes an end time when you press start. In date mode the date and time you enter is itself the end time, interpreted as wall-clock time in your device's time zone. For an event in another country, convert its start time to your zone first.

Once more than a day remains, the display adds a day count in front, such as "100d 09:00:00". When the countdown finishes you hear a short beep, and while it runs the remaining time is shown in the tab title so you can keep an eye on it from another tab.

Date mode counts real elapsed time, not calendar arithmetic. If a daylight-saving change falls inside the countdown, the result is one hour longer or shorter than a naive subtraction of wall-clock times would suggest — which is the correct answer.

Pomodoro: where it comes from and what the evidence says

Francesco Cirillo describes creating the technique in the 1980s, while struggling to concentrate on his university studies, using a tomato-shaped kitchen timer — pomodoro is Italian for tomato. By his account it spread in the 1990s when he coached software development teams.

Research on the technique itself is thin. A 2023 study by Biwer and colleagues followed 87 students at a Dutch university through one real study day. Students given fixed breaks (24 minutes of study and 6 of rest, or 12 and 3) reported less fatigue and distraction and more concentration and motivation than students who decided for themselves when to rest. Self-regulating students studied and rested for longer. Task completion and invested mental effort did not differ between the groups. That points to benefits for mood and efficiency — similar output in less time — rather than evidence that fixed breaks make people produce more.

So there is nothing special about exactly 25 minutes. Here you can set focus from 1 to 180 minutes, short breaks from 1 to 60, long breaks from 1 to 120, and the long-break interval from every 2nd to every 12th focus block.

• Statistics count focus sessions only; breaks are not logged.
• A focus session ended with Skip is not added to your statistics and does not fill a dot toward the next long break.
• Up to 1,000 sessions are kept in this browser, and "today" follows your device's calendar date.

Worked examples

How to use it

1. Pick Stopwatch, Timer or Pomodoro from the tabs at the top.
2. Stopwatch: start, and press Lap whenever you need a split. Timer: enter a duration or choose a date and time, then start.
3. Pomodoro: adjust phase lengths and auto-start if you like, then start.
4. When the exact moment of the alarm matters, keep this tab visible.

Frequently asked questions