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Atomic Clock Time Now

Server-referenced time, and how far your own device clock is from it — measured live in milliseconds.

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More on atomicclock.now

Clock Sync Check Sync check Measure your device's clock against a reference and get a plain verdict — plus how to fix it. UTC Time UTC Coordinated Universal Time to the millisecond, referenced to a synchronised server. TAI vs UTC vs GPS TAI Three time scales, side by side, with the fixed offsets between them.

atomicclock.now does two things: shows you reference time, and tells you how wrong your own clock is. The atomic clock measures your device's drift in milliseconds, the sync check turns that into a verdict and a fix, UTC gives the global reference, and TAI vs UTC vs GPS compares the three atomic scales.

From caesium to your screen

The second is defined by 9,192,631,770 transitions of the caesium-133 atom. National laboratories realise that definition, their clocks are combined into International Atomic Time, UTC is derived from it, and NTP distributes the result across the internet. This page sits at the far end of that chain — accurate to milliseconds, not nanoseconds, and honest about it.

Why your clock drifts at all

Consumer devices keep time with a quartz oscillator whose rate varies with temperature and age, typically drifting a second or so per day if left alone. Automatic synchronisation corrects this continuously, which is why a properly configured machine stays within a few tens of milliseconds.

From our guides

Browse all articles →
The science Atomic Clocks in GPS Satellites: Why Timing Is Position Why satellite positioning is really a timing problem, which atomic clocks each GPS satellite carries, the relativistic corrections applied to them, and what a nanosecond of error costs in metres. 6 min read The science Caesium Atomic Clock vs Rubidium and Optical Clocks How caesium, rubidium, hydrogen maser and optical lattice clocks differ in accuracy, size and cost, where each is used in practice, and why optical clocks will define the second next. 6 min read The science How Atomic Clocks Work: Counting the Caesium Second A plain explanation of how atomic clocks work: the caesium-133 hyperfine transition that defines the second, why atoms keep better time than quartz, and how accurate the best clocks really are. 6 min read 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 Leap Seconds Explained: Why the Clock Sometimes Says 23:59:60 Why Earth's rotation forces the occasional leap second, who decides when one is inserted, the problems it causes for computers, and why the practice is being abandoned by 2035. 5 min read The science National Time Standards: NIST Time and Its Counterparts What national metrology institutes such as NIST, NPL and PTB actually do, how their atomic clocks are combined into International Atomic Time by the BIPM, and how the public gets that time. 5 min read The science Radio Controlled Clocks and WWVB: How They Really Work How the WWVB, MSF and DCF77 longwave time signals reach a radio controlled clock, why reception is best at night, and why these clocks are not atomic clocks at all. 5 min read The science TAI vs UTC vs GPS Time: The Offsets Explained What International Atomic Time, Coordinated Universal Time and GPS time each are, the fixed offsets between them, why satellite time diverges, and which scale your systems should use. 5 min read

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.