Countdown Timer

When you press Start, DateMath notes the moment the timer will end on your device's clock and shows the time left until then, rounded up to the whole second. Because it measures against the clock instead of counting ticks, the timer stays exact when the browser slows down a background tab, and pausing keeps the exact time left. The alarm is set for the end moment and played by your browser, with no sound file to download.

• Your device's local time. DateMath's countdowns read the current time from your device and count to the target in your own time zone. In the US, the standard time zones are set by 15 U.S.C. 261 and daylight saving time by 15 U.S.C. 260a (from 2:00 AM on the second Sunday in March to 2:00 AM on the first Sunday in November); your device applies them. Source: 15 U.S.C. 261, 15 U.S.C. 260a.

DateMath's date math is tested against more than 100 cases worked out by hand. Every worked example and table on the site is computed with the same code the calculators run, so a page and its calculator can't disagree.

The countdown timer counts down a length of time to zero and beeps when the time is up. It is a free web timer for any time countdown online: nothing to install and no sign-up, and it works the same in a phone's browser as on a computer, so you can use it as a time countdown app without downloading one.

What you enter

• Hours, Minutes and Seconds: whole numbers, up to 99 hours, 59 minutes and 59 seconds in total. Minutes and seconds above 59 roll over: 90 minutes is 01:30:00.
• A preset, instead: 30 seconds, 1, 5, 10 or 30 minutes, 1 hour or 24 hours.
• Alarm sound at zero: ticked by default; untick it for a silent countdown.

What you get

The tiles show the hours, minutes and seconds left, and the line above them the clock time the timer will end. Pause stops the countdown with the exact time left, Resume carries on, Reset goes back to the length you set, and Restart runs the same timer again once it has finished. Full screen shows the time countdown across the whole screen, for a classroom, a gym or a stage, and the copy-link button saves the timer's length.

Press a preset or type the minutes. A 10 minute countdown clock shows 00:10:00 and counts down to 00:00:00; a 5 minute countdown suits a short break or a quick task, and a 30 minutes countdown timer a study block or a meeting. Started at 2:15 PM, each timer ends at:

A popular study method works in blocks of 25 minutes with 5-minute breaks: type 25 in Minutes for the work block, and press 5 min for the break.

For a seconds countdown, type the seconds and leave hours and minutes at 0, or press 30 sec. A 10 second countdown clock works for a game or a launch moment, a 30 second countdown timer for a plank or an answer round, and a 60 second countdown for a one-minute speech. You can type 60 or 90 in Seconds as they are: the timer rolls them over, so 90 seconds shows as 00:01:30.

For an hour countdown, press 1 hour or type the hours. The 24 hour countdown clock runs for a full day and ends at the same clock time tomorrow, so the countdown timer for 24 hours suits a sale, a challenge or a waiting period. The timer takes up to 99 hours.

To count down to a date and time instead, such as midnight or a deadline next week, use the day countdown: it ticks down to the moment on your clock, however far away it is.

When you press Start, DateMath works out the moment the timer will end on your device's clock and, ten times a second, shows the time left until then, rounded up to the whole second. The display therefore shows the full length when you start and reaches 00:00:00 only when the time is really up.

• Background tabs. Browsers slow down pages you are not looking at. Because the timer measures against the clock rather than counting ticks, it is still exact when you come back, and the alarm is set for the end moment.
• Phones that lock. A locked phone can pause the page, and then the alarm sounds when you open it again. Keep the screen on for a timer you need to hear.
• The alarm. Three groups of three beeps, made by your browser, plus a vibration on phones that support it. Browsers play a page's sound only after a tap or a click, which pressing Start gives; an iPhone in silent mode can still mute it.
• Pause and resume. Pause keeps the exact time left, to the tenth of a second, so a timer paused and resumed many times loses nothing.

• Cooking: eggs, pasta, a roast that needs basting every 30 minutes.
• Work and study: focused blocks, breaks, timed exams and practice tests.
• Meetings and talks: a 5-minute slot per speaker, a 60 second pitch, a hard stop.
• Workouts: intervals, planks and rest periods between sets.
• Classrooms and games: a timer on the projector in full screen, a 10 second round.

To work out what time it will be after a length of time without running a timer, use the time calculator; to measure the time between two clock times, the hours calculator.