How the alarm clock works
Every quarter of a second the page reads your device’s clock and compares it with the time each alarm is due. Nothing is counted down in the background, so there is no drift to build up. If the page was slowed down while you were in another tab, the very next check still sees that the alarm is due and rings.
Each alarm stores its next ring time. For a repeating alarm the page works out the following occurrence as soon as one has rung. That calculation uses your local calendar, not a fixed 24 hours, so an alarm set for 07:00 is still 07:00 on the morning after the clocks change. Alarms, the 12 or 24-hour choice and your world clock cities are saved in this browser only. Nothing is sent anywhere.
What a browser alarm cannot do
This is a web page, so it can only ring while a browser is running it. If you close the tab, quit the browser or the device is asleep, nothing will sound, and no website can change that. We have tested the case where the clock jumps forward while the page is open, as it does when a device wakes, and that is covered below. We have not tested every phone or laptop sleep setting. For anything you cannot afford to miss, also use the alarm built into your phone.
- Keep the tab open. It may sit in the background, but do not close it.
- Allow sound once. Browsers refuse audio until you have touched the page, so click or press a key after loading it.
- Stop the device sleeping. Tick Keep screen on, or change your power settings, if the machine tends to sleep overnight.
- Background tabs can run late. Browsers slow down timers in tabs you are not looking at, which can delay a check by up to a minute.
Sleep, wake-up and missed alarms
When the page comes back to the front, or the device wakes, it checks every alarm straight away. An alarm that was due within the last 5 minutes rings, and the card says how late it is. Anything older is logged as missed in the Recent list rather than ringing for something you have long since dealt with. A single alarm switches itself off after it has rung or been missed; a repeating one moves on to its next day.
A ringing alarm stops itself after 5 minutes so it cannot go on all night. Snooze rings it again after the delay you set for that alarm, and you can use Edit to change the time, days or sound later.
Using the world clock
Choose a city from the list and press Add city. Each row shows that city’s time, its date compared with yours (Today, Tomorrow or Yesterday), its offset from UTC and how far ahead of or behind you it is, worked out to the minute. Half-hour and quarter-hour zones therefore come out right. A Day or Night label avoids relying on colour alone. The offsets come from the time zone data built into your browser, so they follow daylight saving without being updated by us.
| You want to | Do this |
|---|---|
| Check a call time | Add the other city and read its time and date |
| Use a 12-hour clock | Switch the 12-hour button at the top; every clock follows |
| Show the clock on a spare screen | Press Full screen or the F key |
When another tool fits better
An alarm is for a time of day. For a length of time, such as 20 minutes for a pizza, use the online timer. To measure how long something takes, use the online stopwatch, and for work in focused blocks with breaks, try the Pomodoro timer.
Frequently asked questions
Will the alarm ring if I close the tab or my computer sleeps?
No. A web page can only make a sound while a browser is running it. If you close the tab, quit the browser or the device goes to sleep, nothing will ring. Keep the tab open, tick Keep screen on if you like, and use your phone’s built-in alarm as well for anything important.
Why do I need to click the page before the alarm will make a sound?
Browsers block audio until you have interacted with a page, to stop sites playing sound unasked. Click anywhere on the page, press a key or use the Test sound button once after loading. That unlocks audio for the visit, and the alarm can then sound by itself when its time comes.
What happens to an alarm that was due while my laptop was asleep?
When the page wakes it checks every alarm at once. If one was due in the last five minutes it rings and shows how late it is. If it was due earlier than that, it is listed as missed in the Recent box rather than ringing for something that is long past.
Does the alarm still ring at the right time when the clocks change?
Yes. The next ring time is worked out from your local calendar, so an alarm set for 07:00 stays at 07:00 on the clock after daylight saving starts or ends. If you pick a time that does not exist on the night the clocks go forward, it rings once, an hour later on the clock.
Can I get a notification as well as the sound?
Yes, but only if you ask. Tick Notify me and your browser will ask for permission once. After you allow it, a notification appears when an alarm rings, as long as a browser window is open. If you block it, the box un-ticks itself and a message explains how to allow notifications again.
Where are my alarms and world clock cities stored?
Only in this browser, using local storage on your device. Nothing is sent to our servers, so there is no account and no way for us to see your alarms. They stay after a reload but will not appear on another browser or device, and clearing site data removes them.