Clock Accuracy Check
How closely does this device's clock match the server it just talked to?
Your device clock
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Measuring…
Difference from server time
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Positive means your clock is running behind the server; negative means it is ahead.
How it works
The page asks the server for a small file several times and reads the Date header it sends back. That header describes an instant somewhere between the request leaving and the reply arriving, so the sample with the fastest round trip is used and the server is assumed to have stamped it halfway through — the same reasoning network time protocols use.
The header is only accurate to the whole second, which on its own limits any single reading to about half a second. To sharpen it, the page then polls quickly and watches for the moment the header's value changes. That transition marks the start of a server second precisely, and pins the offset to roughly one polling interval.
What this is not: a comparison against an atomic clock. It measures against the web server that delivered this page, whose own clock may drift. Network delay is also assumed to be symmetric, which is rarely exactly true. Treat the figure as an indication with the stated margin, not a calibration.
Examples
A quick sanity check. Run it once and you get your offset against a network reference with a margin — enough to tell a well-synchronised machine from one that has drifted.
Diagnosing a login failure. Time-based one-time codes and certificate checks fail when a clock is more than a minute out, and a wrong clock is a far more common cause than people expect.
After waking a laptop. Machines that have been suspended for a long time occasionally resume with a drifted clock; this is a fast way to confirm before blaming something else.
Practical notes
The comparison is between your device clock and a network time reference reached over HTTP, so it includes the round-trip latency of that request. The tool measures the bracket rather than assuming the response was instantaneous, which is why the result carries a margin rather than a single exact number.
A browser page cannot match a proper NTP client. NTP uses its own protocol, multiple samples and statistical filtering to reach millisecond accuracy; this gets you to somewhere in the tens of milliseconds, which is enough to spot a clock that is genuinely wrong.
If your clock is off by more than a second or two, fix it at the operating system level rather than by hand — enabling automatic time synchronisation solves it permanently, and every other tool on this site depends on it being right.
Frequently asked questions
Why does the result change between checks?
Network delay varies from request to request, and the margin reflects that. A difference smaller than the stated margin is not meaningful.
My clock is wrong — how do I fix it?
Turn on automatic time in your operating system's date and time settings. That syncs against a proper time server, which is more accurate than anything a web page can do.
Does this use an atomic clock?
No. It compares your device against this site's web server. We would rather state that plainly than imply a precision the method does not have.
How accurate is this check?
To within tens of milliseconds. It measures the round trip rather than assuming an instant response, which is why the result carries a margin instead of a single figure.
Why does my clock drift?
Every crystal oscillator runs slightly fast or slow, and the error accumulates. Automatic time synchronisation corrects it continuously, which is why enabling it is the real fix.
Is this as good as NTP?
No. NTP uses its own protocol and many samples to reach millisecond accuracy. This is enough to tell a healthy clock from a broken one, not to discipline one.
How often should I check?
Rarely, if automatic synchronisation is on. It is worth checking when something time-sensitive fails unexpectedly.
Why does the result vary between runs?
Because network latency varies. Each run measures a fresh round trip, so the margin shifts a little even when your clock has not moved at all.
Can I fix my clock from this page?
No, and no web page can. Enable automatic time synchronisation in your operating system settings — that corrects the drift continuously rather than once.
What counts as a badly wrong clock?
Anything beyond a second or two is worth fixing, and beyond thirty seconds will start breaking things — two-factor codes, certificate validation and log correlation all assume a roughly accurate clock.
Does a wrong clock affect these tools?
Yes, all of them. Every calculation starts from your device clock, so a clock that is a minute out makes every result a minute out in the same direction.