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atomicclock.now — Frequently Asked Questions
Everything about formats, quality, privacy and how Atomic Clock works.
General
Frequently asked questions
Is atomicclock.now free?
Yes — free, no signup, no download.
Is this a real atomic clock?
It is referenced to a synchronised server that traces back to national atomic standards through NTP. The accuracy you can measure in a browser is milliseconds, not nanoseconds.
How do I check if my clock is accurate?
The sync check measures your device against the reference and gives a verdict, plus instructions if it needs fixing.
What is a leap second?
An extra second occasionally inserted into UTC to keep it aligned with the Earth's rotation. The practice is being retired by 2035.
What is the difference between UTC and TAI?
TAI counts atomic seconds without interruption. UTC is TAI adjusted by leap seconds, so it currently runs a fixed whole number of seconds behind.
Atomic Clock Now
Frequently asked questions
What does the drift figure mean?
How far your device's clock sits from the reference. A positive value means your clock is behind; a negative value means it is running ahead.
Why does the figure change between measurements?
Network latency varies. The page takes several samples and keeps the one with the lowest round trip, which is the least distorted estimate.
How accurate can a browser measurement be?
Typically within a few tens of milliseconds. Anything finer needs NTP or PTP running on the machine itself, not a web page.
Clock Sync Check
Frequently asked questions
How do I fix a clock that is wrong?
Turn on automatic time synchronisation in your system settings. On Windows it is Date & time settings, on macOS Date & Time, and on phones it is usually on by default.
Why does my clock keep drifting?
Quartz oscillators are temperature-sensitive and imperfect, so an unsynchronised device typically loses or gains around a second a day. Regular syncing corrects it.
Can a wrong clock break websites?
Yes. A clock off by more than a few minutes can cause HTTPS certificate errors, and one off by more than about thirty seconds can break time-based two-factor codes.
Does this test send any data?
It requests a timestamp from the server and compares it with your clock. Nothing about your device is recorded.
UTC Time
Frequently asked questions
What is UTC?
Coordinated Universal Time, the global reference against which every time zone is defined. It does not change for daylight saving.
Is UTC the same as GMT?
For everyday purposes yes. UTC is the modern atomic standard and GMT is the older astronomical term for essentially the same reference.
How precise is this UTC display?
It corrects your device's clock by the measured server offset, which is typically good to a few tens of milliseconds.
TAI vs UTC vs GPS
Frequently asked questions
What is the difference between TAI and UTC?
TAI counts atomic seconds continuously with no adjustments; UTC is TAI with leap seconds added, so it runs a whole number of seconds behind.
Why does GPS time differ from UTC?
GPS time was aligned with UTC in 1980 and has not added leap seconds since, so the two have drifted apart by exactly the number of leap seconds inserted.
Are leap seconds still being added?
The practice is being retired: a decision has been taken to stop inserting leap seconds, with the change due to take effect by 2035.