How we get the time
An honest explanation of where the time on this site comes from, how accurate it is, and where the limits are.
1. Your device clock
Every clock and timer starts from your phone's or computer's own clock. Devices keep that clock in sync with internet time servers automatically, usually to well under a second. Timers and the stopwatch use your browser's high-resolution timer to measure elapsed time, so they don't drift when you switch tabs.
2. A check against a time server
The main clock pages ask Cloudflare's servers for the current time a few times, keep the fastest answer, and compare it with your device. If your device is off by more than the margin of error, the page shows the corrected time and tells you by how much. Cloudflare's servers keep time with the Network Time Protocol (NTP), which leads back to the atomic clocks that define UTC. If that check can't run, the page falls back to the server's standard web date, which is only accurate to about one second, and says so.
3. Time zones and daylight saving time
Live clocks convert between zones with the time zone rules built into your browser. Tables, city pages and daylight saving dates are generated from the IANA tz database (release 2026e), the same source used by phones, computers and servers worldwide.
4. Cities, maps and the sun
- City search: GeoNames (CC BY 4.0), over 170,000 places with a population of 1,000 or more, each matched to its time zone using timezone-boundary-builder boundaries.
- Major cities: 2,870 cities with their own pages, from Natural Earth via the city-timezones dataset.
- Maps: Natural Earth (public domain).
- Sunrise and sunset: calculated in your browser with standard astronomical formulas, usually within a minute or two of official tables; hills, buildings and weather change what you actually see.
- Moon phase: calculated from the average length of the lunar cycle, so a phase date can be up to a day off from an astronomical almanac.
5. What stays on your device
Dates you type into the age, date and work-hours calculators never leave your device. Your theme, 12/24-hour choice, saved cities and timer settings are kept in your browser's storage. The time check sends nothing about you except an ordinary web request. See the privacy policy.
Limits we want you to know about
- A website can only make sound while its tab is open, and phones may pause pages to save battery. For important wake-ups, also use your phone's built-in alarm.
- Accuracy depends on your connection: on a slow or unstable network the margin of error grows, and the exact time page shows it.
- No website is an atomic clock. We show time that has been checked against servers synced to atomic time.
Questions people ask
Is onlineclock.app an atomic clock?
No website is. Atomic clocks are lab instruments. What this site does is compare your device with time servers that are synced, through the Network Time Protocol, to the atomic clocks behind UTC, and correct for the difference.
Why is my clock a few seconds off?
Your device syncs with a time server only now and then, and some devices have automatic time switched off. Our clocks show the corrected time; to fix the device itself, turn on automatic date and time in its settings.
How accurate is the correction?
The error is at most half the time a request takes to reach the server and come back, usually a few hundredths of a second. The exact time page shows the measured margin for your connection.
Does a time-zone answer depend on my browser?
Partly. Live clocks use the time zone rules built into your browser, which are updated with browser updates. Pages and tables generated on our side use the IANA tz database, release 2026e. In the rare case a country changes its rules at short notice, an out-of-date browser can be wrong until it updates.
Found something wrong? Email [email protected] and we'll fix it.