Countdown, stopwatch and alarm

Countdown: work towards an end

Pick a preset for a short interval, or enter a local date and time for a particular deadline. A ten-minute countdown is useful for a break; a target date suits an event later in the day. Check your device's time zone before entering a deadline supplied by someone in another city.

Stopwatch: measure elapsed time

Start when an activity begins, pause when you need to, and record laps to compare parts of a session. For example, measure three practice runs of a presentation and compare the lap durations before deciding where to shorten it. Reset clears the current timing.

Alarm: use a clock time

Set the time and arm the alarm. It uses your device's local clock and targets the next occurrence of that time. If today's time has passed, it is scheduled for tomorrow. Keep the alarm tab open until it rings.

The five countdown presets are one, five, ten and twenty-five minutes and one hour, and pressing one starts it immediately with that label above the figures. The date field below them takes a calendar date and a clock time in your own zone; leaving the time blank counts to midnight at the start of that date, and a target already in the past shows zero and finishes at once. Pause stores whatever is left rather than remembering the original end, so a countdown resumed after ten minutes away still has the same amount to run. Clear removes the countdown entirely, including its label.

The stopwatch counts in hundredths of a second. Lap is only available while it is running, and every lap records the total elapsed at that press together with the gap since the previous one. The list puts the newest lap at the top and numbers them in the order they were taken. Reset wipes the reading and the laps together, so copy anything worth keeping before you press it.

The alarm takes an hour and minute, nothing finer, and the countdown line beneath it reads down to the second so you can see whether you have time for one more thing. Its state is worked out from the clock rather than stored: armed until the moment arrives, ringing afterwards, and off once you dismiss it. Arming again while it is already armed replaces the target rather than adding a second alarm, and there is one alarm per tab.

Suppose you run a talk three times, pressing Lap at the end of each pass. The list reads from the top down as #3 at 00:14:32.10 with +00:04:28.92 beside it, then #2 at 00:10:03.18 with +00:04:50.78, then #1 at 00:05:12.40 with +00:05:12.40. The left-hand figure is always the time on the clock face when you pressed Lap, counted from the start; the accent-coloured figure beside it is that pass on its own.

The first lap has the same number twice because there is nothing before it to subtract. After that the two columns answer different questions, and the right-hand one is usually the one you want: those three passes took 5:12, 4:50 and 4:28, which is a talk still getting tighter rather than one that has settled. The stopwatch keeps running while you take a lap, so nothing is lost in the press.

Every figure on this page is worked out by subtracting stored timestamps rather than by counting screen updates. That distinction decides what happens when the tab goes to the back. Browsers slow down repeating timers in a hidden tab, often to roughly once a second and sometimes far less, and a device that goes to sleep stops them altogether. Because the remaining time is recalculated from the clock every time the display does update, the number you come back to is correct rather than short by however long you were away.

What suffers is the moment of the alert, not the arithmetic. A countdown or alarm that expires while its tab is hidden or its machine is asleep sounds when the page next gets a chance to notice, which can be seconds or minutes late, and a closed or reloaded page never notices at all. The stopwatch behaves the same way for a different reason: it redraws on animation frames, which stop completely in a hidden tab, so the reading freezes while you are elsewhere and then jumps straight to the right figure when you return.

Two more things can move a result. Changing the system clock moves the target with it, since a countdown is an absolute moment rather than a tally of ticks, so resetting the clock by an hour takes an hour off or adds one. And nothing on this page is saved anywhere: switching between the countdown, stopwatch and alarm discards the tool you leave, and closing or reloading the page clears all three. For anything you must not miss, set an alarm on the device itself, which can ring with the browser shut.

The alert is three short tones about half a second apart, generated by the browser's own audio engine rather than played from a sound file. There is nothing to download and nothing to unmute inside the page, which also means there is no volume control here: the device's own volume is the only one. Many browsers refuse to play any sound until you have clicked or typed on the page at least once, so an alarm armed and then left untouched in a fresh tab can finish in silence.

The habit worth forming is to test it before you rely on it. Start a one-minute countdown, leave the tab where you intend to leave it, and see whether you hear the end of it. The visible state changes either way, with the figures turning accent-coloured and the words Time's up appearing under them, so a silent finish is still obvious to anyone looking at the screen.