How We Calculate
How our timers, clocks and time zones stay accurate.
Timers and the stopwatch
Timers don't count ticks. When you press Start we note the exact end time and, every fraction of a second, show the difference between that end time and now. Browsers slow down background tabs, but the remaining time is always measured against the clock, so it stays correct. The alarm sound is scheduled on your device's audio clock at the moment you press Start, so it rings on time even in a background tab.
Exact time
The exact-time page asks our server for the current time three times, keeps the fastest answer and corrects for half of the round trip. The difference between that and your device clock is the offset shown on the page. Our server keeps its own clock in sync with atomic time servers.
Time zones and daylight saving time
Time zone names, offsets and daylight saving rules come from the IANA time zone database built into your browser, the same data used by phones and operating systems. When a country changes its rules, browsers update that database.
Limits
A web page can only ring while it is open and the device is awake. Accuracy depends on your device clock between syncs, which is why the exact-time page shows how far it is off.
Last reviewed 18 October 2026