Fullscreen Clock

Open it fullscreen and the current time fills the screen

The black preview above becomes the clock as-is. Press Go fullscreen, press F on the keyboard, or double-click the clock, and the whole monitor turns into a clock. Press Esc to close it. The digits scale to the width and height of the screen, so a layout you set up on a laptop keeps the same proportions when you plug into a TV.

The time comes from the clock of the device you are using. When the page opens, it measures how many seconds this device differs from the server time and shows the result in the Clock check box below. If the gap is large, turn on Follow the server time to correct what is shown on screen right away. Your choices are kept in this browser, so the clock looks the same the next time you open it.

Where a fullscreen clock helps

A phone clock is only visible to the person holding it. When a whole room needs to see the same time, putting a big clock on a monitor or TV that is already there is the quickest fix. There is nothing to install, and you just close the tab when the event is over.

• Classroom tests and class periods: Put it on the classroom TV or smart board so students can track the time left on their own. With seconds turned on, the start and stop times are unambiguous.
• Meetings and presentations: On the presenter's second monitor, you can check the time while advancing slides, so talks do not run over.
• Broadcast, streaming and live events: When cues depend on the clock, the host and the crew look at the same digits. The 24-hour format with seconds avoids a.m./p.m. mix-ups.
• Waiting rooms, front desks and shops: A wall monitor can show the date and time for closing times or appointment windows.
• Dark rooms and night shifts: Choose the amber Night color on a black background and lower the brightness to cut glare.

How to use it

1. Pick a style: Choose digital digits or an analog dial. The analog second hand sweeps smoothly once around per minute.
2. Choose what to show: Turn the 24-hour or 12-hour format, seconds, date, AM/PM and the day bar (where now falls between 00:00 and 24:00) on or off. The fewer items you show, the larger the digits become.
3. Match the screen color: Pick Black, White, Night or Green, and set a separate digit color if you need one. Digit size, brightness and digit spacing are adjusted with the sliders.
4. Open fullscreen: Use the button or the F key. Move the mouse or tap the screen and a control bar appears at the top. After 2.6 seconds without input, the bar and the cursor hide again.
5. Adjust on site: In fullscreen, ↑↓ change the digit size and ←→ change the brightness, and the control bar changes the screen color and the style. While Keep the screen awake in fullscreen is on, the screen does not turn off as long as the clock is up.

How far away can you read it: digit height by screen size

Whether something is readable from far away depends on the ratio of its real height to the distance, that is, the angle it covers in your vision. The FAA human factors standard HF-STD-001 says that the character height of critical information should subtend at least 16 minutes of arc (16′) at the farthest viewing distance, and that 22 to 24 minutes is preferred. One minute of arc is 1/60 of a degree.

Required digit height = 2 × distance × tan(angle ÷ 2) · at 22′, about 0.64% of the distance; at 16′, about 0.47%

Feeding the digit height this clock produces on a 16:9 screen into the same standard gives the table below. With the 24-hour format, seconds, date and day bar on (digit size 100%), the digits are about 27% of the screen height; the 12-hour format with AM/PM, the default in U.S. browsers, makes them about 12% smaller. With seconds, date and day bar all off, they grow to about 42%. The minimum-standard distance is about 1.4 times the preferred distance.

The table was measured with the default Mac font, so on a device with a different font the values can differ by a few percent. The readable distance line above recalculates with this device's font and your current settings. If the back row of your classroom is 7 to 8 m (23 to 26 ft) away, even a 32-inch monitor clears the standard comfortably. For a hall deeper than 30 m (98 ft), use a 55-inch or larger screen with seconds turned off, or a projector.

How accurate is the time on screen

This clock shows your device's clock as it is. Computers and phones correct their clocks against internet time servers, but how often and how they do it differs from device to device.

• Windows: A PC that is not joined to a company domain syncs with time.windows.com every 7 days (604,800 seconds) by default. In between, the device clock runs on its own.
• Mac, iPhone and Android: With Set time automatically turned on in the date and time settings, the clock follows network time. With it turned off, the clock you set by hand keeps being used.
• The clock hardware itself: The quartz clock inside a device is typically accurate to within about 0.5 second per day, even in an inexpensive wristwatch, but that difference adds up if it is not synced for a long time.

That is why the page compares against the server time when it opens. The time a server puts in its response (the HTTP Date) is written only down to the second, so a single request cannot tell you anything finer than one second. Instead, the page times its requests to land at the moment the server's second changes, asks several times, and narrows the range the difference can be in. What remains is usually around half the round-trip time of the request, and it is shown on screen with ±.

A wrong time zone does not show up in the check: the device still points at the right moment, and only the displayed hour is off. Make sure the time zone under the Clock check box matches where you are.

Follow the server time corrects only what this page shows and does not touch the device clock. If you need the official time for legal records or attendance, check a clock run by the organization that keeps the national time. In the United States, that is time.gov, the official U.S. time site shown with the logos of NIST and the U.S. Naval Observatory. It shows how far your own clock is off, corrected for network delay.

Recommended settings by situation

Common problems

• The screen turns off after a few minutes: Keeping the screen awake works in Chrome and Edge 84, Safari 16.4 and Firefox 126 or later, and only while the page is fullscreen and the tab is visible. In a browser that does not support it, increase the screen timeout in your device settings.
• Fullscreen does not open on an iPhone: In a browser that does not let web pages go fullscreen, such as Safari on iPhone, the clock fills the browser window instead. Turn the phone sideways to make the digits larger.
• Seconds skipped after visiting another tab: Browsers slow down timers in hidden tabs. The time is corrected the moment you return to the tab, but while the clock is on display, keep its tab in front.
• The digits touch the edge of the screen: If you widened the digit spacing, narrow it again. Even at size 100%, the row of digits is fitted to stay within 92% of the screen width.
• The clock is off by several minutes: Check the difference in the clock check box, fix the device's time zone and automatic setting, then press Check again.

Limits

• The time relies on the device clock and the comparison with the server, so differences inside the measurement error (±) cannot be told apart. Units smaller than a second are not shown.
• The server comparison needs an internet connection. Without one, the clock shows only the device clock.
• How far digits can be read depends on eyesight, lighting, glare on the screen and the viewing angle. The tables give reference values for a straight-on view.
• This clock does not replace official time or legal records.

Sources

• 01FAA HF-STD-001 Human Factors Criteria for Displays (2007), 5.1.8.10Character height of critical information at least 16′ at the farthest distance, 22 to 24′ preferred (p. 29)
• 02RFC 9110 HTTP Semantics, 6.6.1 Date and 5.6.7 Date/Time FormatsThe time the response was generated is written as an HTTP-date down to the second
• 03Microsoft Learn: Windows Time service tools and settingsSpecialPollInterval defaults: 3,600 seconds for domain members, 604,800 seconds for standalone PCs; default server time.windows.com
• 04Apple Support: Set the date and time automatically on MacDate and time are set automatically from a network time server
• 05Android Open Source Project: TimeAutomatic setting under Settings > System > Date & time (Android 11 and lower: "Use network-provided time")
• 06M. A. Lombardi, The Evolution of Time Measurement, Part 2: Quartz Clocks (IEEE I&M Magazine, 2011)Even an inexpensive quartz wristwatch is usually within 0.5 second per day (5 × 10⁻⁶), p. 47
• 07MDN: Screen Wake Lock APIThe lock is released automatically when the document is not visible; request it again when it is visible. Supported versions are in the caniuse wake-lock table
• 08MDN: Fullscreen APIAn element can be made fullscreen only after a user action (click or key press)
• 09time.gov: Official U.S. TimeClocks for U.S. time zones and UTC, with NIST and U.S. Naval Observatory logos; shows how far your clock is off, "corrected for network delay"

Sources checked 2026-10-18. Browser support may widen as new versions are released.

Frequently asked questions

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