How Accurate Is Your Phone or Computer Clock? How to Check and Fix It

With automatic time on, a phone or computer is usually right to within a fraction of a second. With it off, or after weeks offline, the device runs on its own crystal alone and can wander by tens of seconds a month.

Why a clock drifts: quartz, temperature and time#

Your device counts the steady vibrations of a small quartz crystal. No crystal runs at exactly its rated frequency, and the error is quoted in parts per million (ppm). One ppm is 0.0864 seconds per day (computed). A vendor explainer says conventional crystals carry about ±20 ppm at room temperature (Universal Solder), though a given device can do better or worse.

Temperature moves the rate. A common 32.768 kHz watch-style crystal is most accurate near 25 °C and follows a parabolic curve away from it (Universal Solder), so the error grows with the square of the distance from 25 °C and a hot or cold device drifts much faster than one at room temperature. Meinberg, a maker of time-sync equipment, notes that when temperature shifts the crystal's frequency, the system time drifts and sync software must keep adjusting.

This is what a constant error does if nothing ever corrects it (arithmetic, not a measurement):

A laptop with a 20 ppm crystal that stays offline for a month could be nearly a minute out with nothing broken.

How devices correct themselves: NTP, the mobile network and GPS#

The main fix is NTP, the Network Time Protocol (version 4 is RFC 5905). The device asks a time server for the time, notes how long the round trip took and assumes the reply spent equal time in each direction. How Your Phone Knows the Time: Atomic Clocks, Leap Seconds and the Network Behind Them walks through the arithmetic.

Phones have a second route: NITZ, a message in which the mobile network sends local time, date, time zone and daylight saving offset (an early version of the specification, 3GPP). Android's documentation says that from Android 12 the system prefers NTP and falls back to NITZ, because NTP is treated as more dependable.

GPS satellites broadcast time too. A 2021 US Naval Observatory briefing describes GPS time as steered to the Observatory's version of UTC. Whether a given phone sets its clock from GPS depends on the phone; Android's page lists only NTP and NITZ as defaults.

What counts as accurate in practice#

Treat these as ranges, not promises.

• At the source: NIST says its Internet Time Service is accurate on the order of 0.001 seconds, although that varies with network conditions.
• After the network: one NTP vendor's rough guide lists under 10 milliseconds across the public internet and under 1 millisecond on a local network, and blames uneven routes (slower one way) for most error, because NTP assumes equal legs. The fuller NTP guide, How Your Phone Knows the Time: Atomic Clocks, Leap Seconds and the Network Behind Them, cites "tens of milliseconds" as typical on the public internet, so treat 10 milliseconds as a good case.
• On Windows: Microsoft says Windows 10 and Server 2016 or later can deliver 1 second, 50 millisecond or 1 millisecond accuracy under the right operating conditions. Those are targets for configured systems, not a promise about every default install.
• On an ordinary phone or laptop: no source we found publishes a measured figure, so the honest summary is a range, from a few milliseconds to a fraction of a second when sync works.

How to check your clock on Timelive#

Open the exact time page. The chip under the clock and the "Your device clock" card show the result, and the same chip appears on the world clock. Behind it:

1. Your browser asks Timelive's server for its time in milliseconds and times the round trip.
2. It assumes the server read its clock halfway through, so the margin shown is half the round trip. If the first answer takes under 80 milliseconds it stops; otherwise it asks once more and keeps the faster result.
3. If the server cannot be reached, it falls back to the date in an ordinary web response, which has only whole-second resolution, so the margin starts at half a second. If that fails too, the chip says "Offline · showing device time".
4. It repeats every 30 minutes while the tab is visible, and when you return to the tab or reconnect.

Worked examples, computed from the page's code:

"Your clock is exact" appears when the difference is within the margin or 80 milliseconds, whichever is larger. The leading "Exact" describes Timelive's corrected clock, not yours.

Timelive never changes your device's clock, and the result is a snapshot. For a very large offset, the NRC suggests checking date, time zone and daylight saving settings.

How to turn on automatic time#

Menu names change between versions (as of October 2026), so follow the closest match.

• Windows 10 and 11: Start > Settings > Time & language > Date & time, then turn on Set time automatically (Microsoft). Many versions also have a Sync now button under additional settings (Windows Central).
• Mac: Apple menu > System Settings > General > Date & Time, then switch on the automatic date and time option, which syncs the clock to a network time server (Apple).
• iPhone and iPad: Settings > General > Date & Time > Set Automatically (Apple).
• Android: Settings > System > Date & time on many phones, or the Clock app's settings, then turn on automatic date and time. The label varies by phone and Android version (Google).

On Apple devices, a Screen Time passcode or a work profile with restrictions can grey the switch out (Apple). Afterwards, reload the Timelive page to see the new result.

When an exact clock matters#

Software cares when it compares your clock with someone else's.

• One-time codes: authenticator codes (TOTP) are computed from the current time in 30-second steps by default, and RFC 6238 recommends that a verifier allow at most one step for network delay. A phone more than about a minute out is likely to produce codes a server rejects, and servers with no allowance fail sooner. A 20 ppm clock that never synced would be 30 seconds out after about 17 days and a minute out after about 35 (computed).
• Work sign-in: Kerberos, used in Windows domains, accepts a clock difference of 5 minutes by default (Microsoft Learn). At 20 ppm that takes about 174 days (computed).
• Trading: EU rules known as RTS 25 set a 100-microsecond limit from UTC for firms using high-frequency trading techniques, according to explainers from Timebeat and Meinberg. Meinberg ties the 100-microsecond figure to venue operators with gateway-to-gateway latency of 1 millisecond or less, and we did not check the regulation's own text, so confirm the current limits for your case. A 20 ppm crystal drifts 100 microseconds in 5 seconds (computed), and nothing a web page measures comes close.
• Logs and records: an offset larger than the gap between two events can put them in the wrong order when you merge logs from different machines (Unix Time, ISO 8601 and UTC: How Computers Store Time covers timestamps).
• Auctions and deadlines: on most well-built sites the server's clock, not yours, decides when something closes, so a wrong device clock mainly distorts the countdown you see. Check the site's stated rules.

Measuring a duration is a different job: a constant offset cancels out, so a stopwatch needs a steady clock, not a correct one (How to Use a Stopwatch: Laps, Splits and How Accurate Hand Timing Is).

Frequently asked questions

Sources and further reading

1. About Time.gov — NIST
2. Web clock — National Research Council Canada
3. NIST Internet Time Service — NIST
4. Support boundary to configure the Windows Time service for high-accuracy environments — Microsoft Learn
5. Maximum tolerance for computer clock synchronization — Microsoft Learn
6. RFC 6238: TOTP: Time-Based One-Time Password Algorithm — IETF
7. RFC 5905: Network Time Protocol Version 4 — IETF
8. Time source priority — Android Open Source Project
9. USNO briefing to CGSIC, 2021 — GPS.gov
10. Crystal Accuracy, Drift and Temperature Effects in Clock Circuits — Universal Solder