What this measures
The page asks Stopwatch.ca's servers for the current time nine times and notes how long each round trip took. Because a reply travels for half the round trip before you receive it, the time at which the server read its clock is, on average, the middle of the round trip. It keeps the reply with the shortest round trip — the one least distorted by network delays — and compares it with your device's clock.
The result is your device's offset from network time in milliseconds, plus or minus half of that round trip. The server's clocks are kept in step with standard network time, so the figure tells you how far your own clock is from it.
What the result means
- Under about a tenth of a second is excellent. Most phones and computers that sync automatically land here.
- Under a second is fine for everyday use.
- Several seconds or more usually means automatic time is off, the device hasn't synced in a while, or it has been asleep. Meetings, logins and two-factor codes can start failing when a clock drifts far enough.
Fixing a clock that's off
Turn on automatic date and time and set time zone automatically in your device's settings, then measure again. On a computer, a manual sync from the date-and-time settings does the same thing. If your offset is whole hours, the time zone, not the clock, is wrong.
Limits to keep in mind
The method assumes the network is about as fast in each direction; if it isn't, the true offset can differ by up to half the round trip, which is why the uncertainty is shown. Browsers also deliberately blur their timers slightly. Treat the number as accurate to a few tens of milliseconds at best — enough to tell a healthy clock from a drifting one, but not a substitute for a laboratory time standard.
To watch your clock run, see the online clock; to compare cities, the world clock; and for how precise the stopwatch itself is, how accurate is an online stopwatch?
Questions
How accurate is my computer's clock?
Computers and phones that sync automatically over the internet are usually within a few tens of milliseconds of network time. Without syncing, a clock can drift by seconds a week.
Why does it say ± some milliseconds?
The reply from the server takes time to arrive and the trip may not be symmetrical, so the offset is uncertain by up to half the round-trip time. The page uses the fastest reply to make that as small as it can.
My clock is off by exactly an hour. Why?
Almost certainly the time zone or daylight saving setting. Turn on set time zone automatically and measure again.
Does the page send my data anywhere?
It only asks Stopwatch.ca for the time. Nothing about you is stored.
Is network time the same as atomic time?
Network time services keep clocks aligned with the time scales maintained by national laboratories, which are based on atomic clocks. The server here keeps its clocks synchronised over the network time protocol; this page treats that as the reference.