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Is My Computer Clock Accurate?

Measure your device's clock against a reference and get a plain verdict — plus how to fix it.

A wrong clock breaks more than punctuality: it breaks HTTPS certificate checks, two-factor codes, calendar invitations and log correlation. This clock sync check measures your device against a synchronised reference and gives a plain verdict — accurate, slightly off, or needs fixing.

What each verdict means

Under about a quarter of a second, your clock is synchronised and there is nothing to do. Up to a couple of seconds is worth a sync but rarely breaks anything. Beyond that, two-factor authentication codes and certificate validation start failing, which is usually how people discover the problem in the first place.

Fixing it takes about a minute

Every modern operating system can set the clock automatically; the setting is occasionally switched off or pointing at an unreachable server. Step-by-step instructions for Windows, macOS, Linux, iOS and Android are in how to sync your computer clock, and the reasons clocks wander in the first place are in why computer clocks drift.

Guides about Clock Sync Check

Fixing your clock How to Sync Your Computer Clock on Any Device Step-by-step instructions to sync your computer clock on Windows, macOS, Linux, iOS and Android, plus how to choose a time server and diagnose a sync that refuses to work. 6 min read The science What Is NTP? The Network Time Protocol Explained How the Network Time Protocol measures and corrects clock offset, what the stratum hierarchy means, the accuracy to expect over the internet, and why NTP slews rather than jumping. 6 min read Fixing your clock Clock Drift: Why Computer Clocks Lose Time Why every computer clock drifts: quartz tolerance in parts per million, the strong effect of temperature, what virtual machines and sleep add, and how synchronisation corrects it. 6 min read

More atomicclock.now pages

Atomic Clock Now Server-referenced time, and how far your own device clock is from it — measured live in milliseconds. UTC Time Coordinated Universal Time to the millisecond, referenced to a synchronised server. TAI vs UTC vs GPS Three time scales, side by side, with the fixed offsets between them.

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.