How it works

Your browser cannot talk to time servers directly, so the time travels in two steps: from European atomic clocks to our server, and from our server to you. Each step is measured and corrected.

Step one: our server and the atomic clocks

Every five minutes our server asks these national time institutes for the time, using the Network Time Protocol (NTP):

• PTB, Braunschweig (DE), ptbtime1.ptb.de
• INRIM, Turin (IT), ntp1.inrim.it
• GUM, Warsaw (PL), tempus1.gum.gov.pl
• UFE, Prague (CZ), time.ufe.cz

Each institute maintains its country’s official time with caesium atomic clocks, compared continuously with the world’s other laboratories to form Coordinated Universal Time (UTC). Several samples are taken from each one, and only the fastest exchanges are kept, because a quick round trip leaves little room for error. Any institute that disagrees with the others is set aside, and the rest are averaged. The result is a correction for our server’s own clock, usually accurate to a few milliseconds.

Step two: our server and your browser

When you open the page, your browser asks our server for the time eight times in a row. For each request it records four moments: when the request left, when our server received it, when our server replied, and when the reply arrived. From these four numbers it calculates two things: how long the internet connection took, and how far your device’s clock is from the true time. This is exactly how NTP itself works.

The requests with the shortest round trip are trusted most. The clock keeps checking quietly once a minute, and again whenever you return to the tab.

How accurate is it?

The accuracy shown on the clock page is an honest upper limit, made of three parts:

• Half of the shortest network round trip. The method assumes the request and the reply take equally long. If they do not, the error can be at most half the round trip.
• The uncertainty of our server’s own correction, measured in step one.
• One screen refresh of about 8 to 16 ms, because a display can only change between frames.

On a typical European home connection this comes to about ±15 to ±40 ms. On mobile data it can be higher. The true error is normally well inside the figure shown.

Your device clock

Phones and computers synchronise their own clocks too, but often only once a day or week, and some use servers far away. The page compares your device’s clock with the true time and tells you the difference. If it is more than a second, check that automatic time is switched on in your device’s settings.

Pips and flashes

The minute pips are scheduled in advance with the browser’s audio engine rather than played on a timer, and the page subtracts the audio delay the browser reports. Bluetooth speakers and headphones add their own delay of 100 to 300 ms, which browsers usually cannot see, so use the device’s own speaker when precision matters.

Daylight saving time

The clock follows your device’s time zone. When the change between summer and winter time is close, the clock page shows exactly when it happens.

Privacy

Fonts are served from this site, not from Google or any other third party. The clock needs no cookies. Your display choices, such as the 12-hour format, are remembered only in your own browser. More in the privacy policy.