Fullscreen Digital Clock (24-Hour)

What Is a Fullscreen Digital Clock?

A fullscreen digital clock is a web page that fills the entire screen with a large, easy-to-read time display. It is meant to run on a second monitor, a wall-mounted display, a classroom projector, or any screen where the time needs to be visible from across a room. The clock defaults to 24-hour time and updates once per second.

24-Hour Time

24-hour time, sometimes called military time, counts the hours of the day from 00 to 23. Midnight is 00:00:00, noon is 12:00:00, and the hour after noon is 13:00:00 rather than 1:00:00. Because every hour of the day has its own number, there is no need for an AM or PM label, and no risk of confusing a morning appointment with an evening one. This clock shows 24-hour time by default, in the format HH:MM:SS, with the hour, minute, and second each padded to two digits.

12-Hour Time

Many people are more used to reading a 12-hour clock, where the hour resets after 12 and an AM or PM marker shows which half of the day it is. This tool includes a toggle for 12-hour mode. In that mode the hour runs from 1 to 12 and is not zero-padded, so five past one in the afternoon reads 1:05:05 PM, not 01:05:05 PM.

Two moments cause confusion on a 12-hour clock: midnight and noon. There is no "0 o'clock" on a 12-hour dial, so by convention midnight is written 12:00:00 AM and noon is written 12:00:00 PM. The clock follows this convention.

Display Options

The page has four toggles, all of which reset to their default with the Reset button:

• 12-hour format: switches between 24-hour time (default) and 12-hour time with an AM/PM marker.
• Show seconds: shows or hides the seconds field, leaving HH:MM or h:MM AM/PM.
• Show date: shows or hides a full date line, such as "Monday, January 15, 2024", below the time.
• Show time zone: shows or hides a small label with the browser's detected time zone, such as Europe/Berlin (CET). The zone is read from the browser, not typed in, so it always matches the device the clock is running on.

Using Fullscreen Mode

The "Enter Fullscreen" button uses the browser's Fullscreen API to expand the clock to fill the whole screen, hiding the browser's address bar and tabs. Clicking the button again, or pressing Esc, exits fullscreen and returns to the normal page. The button label switches to "Exit Fullscreen" automatically once the page is fullscreen, including when fullscreen is exited with Esc rather than the button.

Not every browser or embedded view supports the Fullscreen API. Some older browsers, some in-app browsers, and iframes that block fullscreen will not honor the request. If that happens, the page shows a message instead of failing silently.

Worked Example

Suppose the local time is 5 seconds past 1:05 in the afternoon.

• In the default 24-hour mode, the clock reads 13:05:05.
• With the 12-hour toggle turned on, the same instant reads 1:05:05 PM.
• With seconds turned off, the two modes read 13:05 and 1:05 PM.

At midnight, the 24-hour clock reads 00:00:00 and the 12-hour clock reads 12:00:00 AM.

Frequently Asked Questions

What is 24-hour time? 24-hour time numbers every hour of the day from 00 to 23, so each hour is unique and no AM or PM label is needed. It is the default format used by this clock.

Why does a 12-hour clock show 12:00 AM for midnight? A 12-hour clock has no zero hour. By convention, the hour after 11 PM is labeled 12 AM rather than 0 AM, marking the start of a new day, and the hour after 11 AM is labeled 12 PM, marking midday.

Why is the fullscreen button not working? Some browsers and embedded views, such as certain in-app browsers or sandboxed iframes, do not support the Fullscreen API or block it outside a direct user click. The page shows a message when a fullscreen request fails or is unsupported.

Does the clock show my time zone or a fixed one? It always shows the local time of the device it runs on, taken from the device's clock and settings. Turning on the time zone toggle labels that local time with the browser's detected IANA time zone, such as America/New_York, and its current abbreviation, such as EST or EDT.

Can I bookmark a specific set of display options? Yes. Each toggle is stored in the page's URL, so a link copied while the seconds, date, 12-hour, or time zone options are set a certain way will reopen with those same options.

Does the time keep running if I leave the tab open? Yes. The display updates once per second for as long as the page stays open, whether or not fullscreen mode is active.