How the World's Clocks Stay Accurate — Atomic Time, NTP, and Leap Seconds
Your phone shows the right time to within a fraction of a second, every day, without you ever setting it. Your oven drifts a minute a month. The difference is a global chain that starts with atoms and ends with a quiet network protocol from 1985.
Every clock drifts — the question is what corrects it
Almost every consumer device keeps time with a quartz crystal vibrating at (nominally) 32,768 times per second. Quartz is cheap and good, but temperature and manufacturing tolerances make each crystal run slightly fast or slow — typically by a few seconds per month. That's why the standalone clocks in your life (oven, car, wall clock) wander, while anything with an internet or cellular connection stays right: connected devices are constantly being nudged back toward a reference.
The reference: atomic time
Since 1967, the second has been defined by the atom: one second is exactly 9,192,631,770 oscillations of the microwave radiation that flips the state of a caesium-133 atom. Caesium fountain clocks realize this definition so well that they would drift by about one second in a hundred million years.
No single clock is trusted alone. The BIPM (International Bureau of Weights and Measures, near Paris) combines readings from over 400 atomic clocks in national labs worldwide into a weighted average called TAI (International Atomic Time). TAI is the steadiest timescale humanity produces — but it ignores the Earth.
UTC and the leap second problem
The Earth's rotation is slightly irregular and, over long periods, slowing. If clocks ran on pure TAI, atomic noon and solar noon would slowly drift apart. The compromise is UTC: it ticks with atomic regularity, but whenever it strays too far from Earth-rotation time, a leap second is inserted. Since 1972 this has happened 27 times — most recently at the end of 2016 — which is why UTC is currently exactly 37 seconds behind TAI.
Leap seconds are famously disruptive for computer systems (a minute with 61 seconds breaks a lot of software assumptions), and in 2022 the General Conference on Weights and Measures resolved to stop inserting them by 2035. Until then, they remain a scheduled-on-short-notice quirk of the world's official time. For what UTC means in daily use — and how it differs from GMT — see our UTC vs GMT guide.
GPS: the time signal from space
Every GPS satellite carries atomic clocks, and a position fix is really a time calculation — your receiver compares how long each satellite's signal took to arrive. As a side effect, GPS is one of the best time distribution systems ever built: a cheap receiver gets time to within nanoseconds.
One quirk: GPS time does not observe leap seconds. It has run continuously since 1980 and is currently 18 seconds ahead of UTC; the satellite message includes the current offset so receivers can convert. Data centers, cell towers, and financial exchanges all use GPS-disciplined clocks as their local reference.
NTP: how the internet agrees on the time
The Network Time Protocol, created by David Mills in 1985, is one of the oldest internet protocols still in universal use. It works by layering servers in strata:
- Stratum 0 — the reference hardware itself: atomic clocks and GPS receivers.
- Stratum 1 — servers wired directly to stratum 0 hardware.
- Stratum 2, 3, … — servers that sync to the layer above and serve the layer below.
A client sends a timestamped request, the server replies with its own timestamps, and the client uses the round-trip timing to estimate both the network delay and its own clock error — then gently slews its clock rather than jumping it. Over the public internet NTP typically achieves accuracy of a few milliseconds, which is far better than any human can perceive.
Your devices do this automatically: Windows uses time.windows.com, Apple devices use time.apple.com, most Linux systems use the volunteer pool.ntp.org project, and phones additionally receive time from the cellular network itself. The whole system is invisible precisely because it works.
So why is some clock in your life still wrong?
- It has no network connection — ovens, cars, wall clocks. They're pure quartz and will drift forever; set them against a synced source like gworldtime.
- Its time zone is set wrong. A clock can be perfectly synced to UTC and still display the wrong hour if the zone or DST setting is off — a very common cause of "my computer is exactly one hour wrong." Our time zone guide explains how zone rules are applied.
- Its battery-backed clock died — a PC that boots to 2010 has a flat motherboard battery; it forgets the time whenever power is cut, until NTP rescues it after boot.
How gworldtime uses all of this
When you open gworldtime, it doesn't just trust your device's clock. It measures the offset between your device and a network time reference (the same request/response technique NTP uses), then renders every one of its 337 cities by applying that corrected UTC to each city's IANA time zone rules. The result: even on a device whose clock is minutes off, the globe shows the true current time everywhere.
Read next: UTC vs GMT — what's the difference? · How time zones work · Daylight Saving Time explained