Clock Help Documentation

A complete guide to Clock, from the first alarm to the world clock.

Last Updated: November 10, 2026

1. Getting Started

The tool has five tabs: Clock, Alarms, Timer, Stopwatch and World. The Alarms tab shows how many alarms are on; a small dot on the Timer or Stopwatch tab means it is running. The line beside the tabs shows your next alarm and how long until it rings, and under it the timer's time left, a running stopwatch, a missed alarm or your time zone. Sound and settings and How it works sit beside the tabs. The round button at the top right switches the site between light and dark.

Keep the page open in a tab while you rely on an alarm or the timer, and click it once after it opens so it may make sound.

2. The Clock Tab

The time in large figures, with the seconds and AM or PM beside it, the date, and your time zone with its offset from UTC, for example Kolkata, Asia (UTC+5:30). The analogue clock beside it moves its hands with the time. Under them:

• 12 hour or 24 hour: the format of every time on the page, alarms and cities included.
• Seconds: shows or hides the seconds and the second hand.
• Clock face: Classic, Numbers, Marks only, Minimal, Bold or Accent.
• Full screen: see the next section.

3. Full Screen

Full screen (or F on the Clock tab) fills the screen with the time and the date, in the site's light or dark theme, and asks the browser for its full screen view. Move the mouse or tap to show Exit full screen; Esc or F closes it. An alarm or the timer rings over it.

4. Setting an Alarm

On the Alarms tab press New alarm. The form opens with the time one hour from now.

• Time: the hour and minute.
• Label: up to 40 characters, such as Wake up or Call home; without one the alarm is called Alarm.
• Repeat: the days Mon to Sun, or Weekdays, Weekends, Every day or Once. With no day chosen the alarm rings once.
• Sound: Chime, Beep, Bell, Digital or Soft; Listen plays it.
• Rings: how long until it rings, and then on which days.

Save alarm adds it to the list, which is sorted by time. Each alarm shows its time, label, days and how long until it rings, with a switch to turn it off and on, a pencil to change it and a bin to delete it (Undo in the message). An alarm saved for the minute it is now rings on its next day, as the Rings line says. Up to 50 alarms are kept.

5. When an Alarm Rings

A window covers the page: a bell, the time, the label and the days, and two buttons, Snooze 5 min and Dismiss. Esc dismisses. The alarm's sound repeats until you choose; after five minutes of ringing the sound stops and the window says so, staying until you close it. Alarms due at the same moment ring together, the others named under the first. A one-off alarm turns itself off when it rings; a snooze turns it back on until the snoozed ring. On most Android phones the phone also vibrates.

When the timer reaches zero, the same window says Timer finished, with Dismiss, and the timer goes back to its length.

6. When Alarms Can Ring

• The page must be open in a tab, and the device awake. A closed tab, a laptop with its lid shut or a phone that has put the browser to sleep cannot ring.
• Click the page once. Browsers keep a page silent until you have clicked, tapped or pressed a key on it since it opened. Until then, with an alarm on or the timer running, a note above the tabs asks you to.
• Other tabs are fine. Browsers wake a hidden tab less often, so the page sets one extra timer for the next alarm when you switch away, and every check compares with the clock. A hidden tab can still ring a little late; the window then says how late.
• Missed alarms. When the page is open again, an alarm found up to ten minutes late still rings. One found later is marked Missed at its time on the Alarms tab, the line beside the tabs counts it, and a message says so. The mark goes when the alarm next rings or when you change it.

For an alarm you must not miss, set one on your phone as well.

7. Summer Time and Alarms

Alarms follow your device's time zone. On the night the clocks go forward, one hour never happens: an alarm inside it, such as 2:30 when the clocks jump from 2:00 to 3:00, rings as many minutes later as the clocks jumped, at 3:30. On the night they go back, one hour happens twice: an alarm inside it rings once, the first time. The Rings line and the list show these times in advance. India has no summer time, so there this never happens.

8. The Timer

Set the hours (0 to 99), minutes and seconds (0 to 59) in the boxes. The plus and minus buttons change the hours by one and the minutes and seconds by five. A preset (1, 3, 5, 10, 15, 25 or 45 minutes, or 1 hour) sets the time and starts at once.

• Start begins the countdown; the ring empties and the line under the time says when it ends. The tab's title shows the time left.
• Pause stops it (the ring turns amber), Resume carries on, +1:00 adds a minute (up to 99:59:59 in all), Reset puts the time back.
• At zero the ring turns green and the timer rings with the sound chosen in Sound and settings. Start again runs the same length once more.

The timer keeps its end time, so it keeps counting while the page is closed. Opened again less than a minute after it ran out, it rings; later, a message says when it ended.

9. The Stopwatch

Start begins counting in hundredths of a second, Stop stops it, and Resume carries on from the same time. Lap (only while it runs) adds a lap; Reset (only when it is stopped) clears the time and the laps. Each lap shows its own time and the total, newest first, with the lap in progress at the top. From two laps on, the fastest lap is marked fastest in green and the slowest slowest in red. Up to 1,000 laps are kept. A reload carries on from the right time.

10. The World Clock

Type a city or a time zone in the box (the list under it suggests every zone your browser knows) and press Add, or tap a popular city: New York, London, Mumbai, Tokyo, Sydney or Dubai. The first card, Your time, is your own zone. Each card shows:

• the time there (with seconds when Seconds is on) and the date;
• the difference from your time, such as +3:30 h, and the day when it differs: tomorrow, yesterday, or, between the zones furthest apart, the day after tomorrow or the day before yesterday;
• a sun for daytime hours there (6 am to 6 pm) or a moon for the night hours, a quick guide rather than the real sunrise and sunset.

The cross on a card removes it (Undo in the message). Up to 50 cities are kept. Well-known cities that are not zone names are understood too, such as Mumbai, Delhi, Bengaluru, Beijing, San Francisco or Washington.

11. Time Zone Names

Every zone's rules come from the tz database built into your browser. A few zones were renamed in it, but some browsers still give the old name: Asia/Calcutta for Asia/Kolkata, Europe/Kiev for Europe/Kyiv, Asia/Saigon for Asia/Ho_Chi_Minh, Asia/Rangoon for Asia/Yangon and others. The page always shows the current name. Fixed-offset zones such as Etc/GMT+5, which the database names with the sign reversed, are shown as UTC-5.

12. Sound and Settings

• Sound: the alarm volume (0 to 100%, with Test) and the sound for new alarms and the timer.
• Notifications: see the next section.
• Keyboard shortcuts: the list of keys.
• Your data: Save a backup, Restore a backup and Delete everything.

Every sound is made in your browser; nothing is downloaded.

13. Notifications

Turn on Show a notification when an alarm or the timer rings and allow notifications when your browser asks. While this page is open and you are in another tab or window, a message then appears when something rings. If notifications are blocked for the site in the browser's settings, or the browser has none, the switch turns itself off and says why. Nothing is sent from a server: the message is shown by your own browser.

14. Backup, Restore and Delete Everything

• Save a backup: your alarms, cities and settings in one small file, clock-backup.json.
• Restore a backup: reads such a file (up to 2 MB, also from the earlier version of Clock), checks every value first, and asks before it replaces the alarms, cities and settings here. Restored alarms count from the moment they are restored, so a time already past does not ring.
• Delete everything: removes all alarms, cities and settings this tool keeps in this browser and stops the timer and the stopwatch, after you confirm. The rest of the site is left alone.

15. Keyboard and Accessibility

Every button and field works with the keyboard. The tabs are a tab list: the left and right arrows, Home and End move between them. While you use the tool and are not typing, 1 to 5 switch tabs, Space starts and pauses the timer or the stopwatch, L adds a lap, R resets and F opens the full screen clock; Esc dismisses an alarm and closes windows, the full screen clock and the alarm form. Elsewhere on the page these keys do what they always do. The ringing window takes the keyboard focus on its Dismiss button and is announced to screen readers. With reduced motion set on your device, the hands, the ring and the bell do not animate.

16. Privacy

Clock works in this browser, and nothing you do in it is sent anywhere: no upload, no account and no server-side storage. Every sound is made on your device, and every city's time comes from your browser's own time zone rules. Like every page on ZeroDataUpload, the page shows advertising and counts visits; the cookie banner explains the choices and the Privacy Policy explains both. What you do in the tool, such as your alarm labels, is never part of them, and the tab's title shows a running timer or stopwatch but never an alarm's label.

17. How It Was Checked

Before this page was published, the engine behind the tool was checked against references written from the rules on this page, with the test values made first: 595,789 cases in 13 groups. They include every time format, when each alarm rings next over a whole year in eight time zones (summer time changes, the hour skipped and the hour repeated, days of the week and one-off alarms), alarms checked at regular and very irregular moments (a hidden tab, a device asleep for hours) so that each rings exactly once or is marked missed, the timer and the stopwatch through random presses, every time zone the browser knows against the tz database at the moments around each change, the renamed zones, the city search, and saved data and backups made to break the tool. Then the page was tried at four screen widths in both themes.

18. Troubleshooting

• An alarm rang without sound. Click the page once after it opens; check the volume in Sound and settings and that the device is not muted.
• An alarm did not ring. The tab was closed or the device asleep; the Alarms tab marks it as missed. Keep the tab open and the device awake, and set one on your phone for anything important.
• An alarm rang late. The tab was out of sight and the browser woke it late; the ringing window says how late.
• No notification appeared. Allow notifications for the site in the browser's settings and turn the switch on again; some systems hide them in a do-not-disturb mode.
• A city cannot be found. Type a bigger city in the same time zone, or the zone's name, such as Europe/Berlin.
• My alarms are gone. They are kept per browser and device; a private window or cleared browser data starts afresh. Restore a backup file to bring them back.

19. Support

For questions the FAQ does not answer, use the contact form.