Clock Accuracy Checker
Find out whether your computer or phone shows the right time. The checker compares your device clock with a time server, shows the difference in milliseconds and keeps an exact, corrected clock running below.
Last updated:
Measurement samples
| # | Round trip | Offset estimate | Range |
|---|---|---|---|
| No samples yet. | |||
Your time settings
Only a small time request is sent to the selected server; nothing about you is stored. Accuracy is limited by network delay, shown as ± in the result.
How to use the Clock Accuracy Checker
- Open the page: the check starts automatically and takes a few seconds.
- Read the verdict: it says how far your clock is ahead or behind, with a rating from Excellent to Needs fixing.
- Look at the ± value: it is how precise the measurement is, based on your network round-trip time.
- Use the exact clock: the big clock is already corrected for your device's error, with milliseconds, UTC and Unix time.
- Fix a wrong clock by turning on automatic time in your system settings, then press Check clock again to confirm.
The formula behind the offset
For each request the checker records the device time when it was sent (t0) and when the reply arrived (t1), and reads the server time T from the reply. Assuming the request and the reply take the same time over the network:
- Round-trip time = t1 − t0
- Server − device offset = T − (t0 + t1) ÷ 2
- Maximum error = ± round-trip time ÷ 2
Example: you send at 12:00:00.100 by your clock, the reply arrives at 12:00:00.140 and the server says 12:00:00.320. The midpoint is 12:00:00.120, so the server is 200 ms later than your device: your clock is 0.200 s behind, give or take 20 ms.
How far off is too far?
| Difference | Rating | What it means |
|---|---|---|
| Under 50 ms | Excellent | Normal for a device with automatic time sync. Good enough for almost anything. |
| 50–500 ms | Good | Fine for everyday use; may matter for online auctions, ticket drops or timing sports. |
| 0.5–2 s | Fair | Sync is probably infrequent. A manual sync will fix it. |
| Over 2 s | Needs fixing | Automatic time is likely off. Errors of 30 s or more can make one-time login codes fail. |
Why device clocks drift
Computers and phones keep time with a quartz crystal oscillator. A typical crystal is accurate to about 20 to 50 parts per million, which adds up to roughly 2 to 4 seconds per day if the clock is never corrected, and temperature changes make it worse. That is why operating systems regularly re-synchronize with internet time servers. If your clock is far off, check that automatic time is on, that the correct time zone is selected (a wrong zone shows a whole-hour error, which this checker does not count because it compares absolute time), and that a firewall is not blocking time sync.
Frequently asked questions
How does the clock accuracy checker work?
It sends a few small requests to a time server and notes your device time just before and just after each one. The server time is assumed to have been read halfway through the round trip, so offset = server time − (send time + receive time) ÷ 2. This is the same basic idea that NTP, the Network Time Protocol, uses. The request with the shortest round trip gives the most precise answer.
How accurate is the result?
The uncertainty is about half the network round-trip time, usually 5 to 50 milliseconds on a normal connection, and it is shown as ± in the result. When only the 1-second HTTP Date header is available, requests are timed to land on the server's second boundary to narrow the range down to a few tens of milliseconds.
Is this an atomic clock?
No website can show an atomic clock directly. Internet time servers are synchronized to atomic clocks (for example via GPS or national time labs), and this page corrects your device time using such a server, so the big clock is typically within a few hundredths of a second of official time.
How accurate should my computer clock be?
With automatic time sync turned on, Windows, macOS, Android and iOS usually keep the clock within about 10 to 100 milliseconds of official time. A difference of several seconds or more means automatic sync is off or failing. Errors of minutes can break logins, two-factor codes and HTTPS connections.
How do I fix a clock that is ahead or behind?
Windows: Settings > Time & language > Date & time, turn on Set time automatically and press Sync now. macOS: System Settings > General > Date & Time, turn on Set time and date automatically. iPhone and Android: Settings > General (or System) > Date & time, turn on Set automatically. Then run the check again.
What does ahead or behind mean?
Ahead means your device shows a later time than the real time (offset is positive), so things happen early by your clock. Behind means your device shows an earlier time than the real time (offset is negative).
Does the check send my data anywhere?
Only a tiny time request is made, to this website's own server or, if you choose it, to worldtimeapi.org. No personal data is sent or stored, and your result is not saved anywhere.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.