If you've ever asked "what time is it exactly?", you may have been surprised to learn there are several answers. They all tick at nearly the same speed, but they start from different points and treat leap seconds differently. Here is a plain-English map.
Coordinated Universal Time is what the world uses for civil timekeeping. It is based on atomic clocks but adjusted with leap seconds so it stays close to the Earth's rotation. Time zones are defined as offsets from UTC. You can view it directly on our UTC page.
International Atomic Time is a continuous count of seconds with no leap seconds. Since 1972, leap seconds have been added to UTC and not to TAI, so TAI is now 37 seconds ahead of UTC. Scientists use TAI when they need a scale that never jumps.
Every GPS satellite carries atomic clocks, and the system keeps its own time scale. It began at midnight on 6 January 1980, when it matched UTC, and it has not added leap seconds since. GPS time is therefore 19 seconds behind TAI, and currently about 18 seconds ahead of UTC. GPS receivers get the leap-second offset from the satellites' navigation messages and convert to UTC for display, which is why your phone shows normal time.
Interestingly, satellite clocks have to be corrected for Einstein's relativity. Because the satellites are higher in Earth's gravity and moving fast, their clocks would run about 38 microseconds per day faster than clocks on the ground if left alone. Engineers adjust for that in advance. Without the correction, position errors would grow by kilometres each day.
Most computers count time as the number of seconds since 1 January 1970, called Unix time. It deliberately ignores leap seconds. On a leap second day, systems must either repeat a second, or "smear" the extra second over hours. That is why leap seconds have caused outages in the past.
Your computer or phone has a small quartz oscillator, much like a watch. Left alone, it can drift by a second or two per day. To fix that, devices use NTP or their mobile network to correct the clock regularly.
| Scale | Leap seconds? | Offset from UTC (currently) | Typical use |
|---|---|---|---|
| UTC | Yes | 0 | Civil time, time zones |
| TAI | No | +37 s | Science, metrology |
| GPS time | No | +18 s | Satellite navigation |
| Unix time | Ignored | Tracks UTC, with special handling | Software |
| Device clock | Depends | Usually within milliseconds if synced | Everyday use |
The offsets can change if leap seconds are added, but none have been added since 2016, and they are due to be phased out.
For daily life, the clock on your phone is fine. Networks keep it within a fraction of a second of UTC. For setting a wristwatch by eye, any synchronised clock will do, since your reaction time is about 0.2 seconds. For engineering, finance or science, you'll want a source with a documented uncertainty, such as a GPS-disciplined clock or a good NTP server.
A website's clock depends on your device's time and on the network delay to the server that corrects it. That is fine for most purposes. When you need better than about a tenth of a second, use a dedicated time source.
Compare the time in different places on our world clocks, or pick a city like Berlin, Nairobi or Auckland to see its UTC offset. To use precise time in practice, read our guide to calibrating a watch, and to learn how the reference is built, see how atomic clocks work.