Atomic Clock Time Now — Check Your Clock's Drift

The clock above is referenced to a synchronised server rather than to your own device, and the reading underneath tells you how far apart the two are. That difference is your device's clock drift. The measurement takes several round-trip samples and keeps the fastest, halving the round trip to estimate the one-way delay — the same basic idea as the Network Time Protocol, at much coarser resolution.

What 'atomic clock' means here

No web page contains an atomic clock. The chain runs from caesium and optical standards at national laboratories, through International Atomic Time and UTC, out over NTP to ordinary servers, and finally to this page. Each link adds a little uncertainty, so treat the reading as good to a few tens of milliseconds rather than to the nanosecond. How the real clocks work is in how atomic clocks work.

Why a few milliseconds is a good result

An unsynchronised quartz clock can drift by a second or more per day. If this page says you are within a couple of hundred milliseconds, your operating system is syncing properly and there is nothing to fix. If it says several seconds, something is wrong — the sync check explains what.

Guides about Atomic Clock Now

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 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

More atomicclock.now pages

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

Is this a real atomic clock?

It is referenced to a server whose clock is synchronised by NTP, which traces back to national atomic standards. The page itself contains no atomic clock, and accuracy is measured in milliseconds rather than nanoseconds.

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.