Time scales and leap seconds
There is more than one “correct” time. Atomic time, navigation satellites, astronomy and computers each count time in their own way. Here they are side by side, running live on our atomic-referenced clock.
| Scale | Now | Difference from UTC | Used for |
|---|---|---|---|
| UTC | 0 s | Civil time worldwide; the basis of every time zone | |
| TAI | +37 s | International Atomic Time; the pure atomic count, without leap seconds | |
| GPS time | +18 s | GPS satellites; equal to TAI − 19 s since 1980 | |
| Galileo (GST) | +18 s | Europe’s navigation system; kept equal to GPS time | |
| BeiDou (BDT) | +4 s | China’s navigation system; equal to TAI − 33 s since 2006 | |
| TT | +69.184 s | Terrestrial Time, used in astronomy; TAI + 32.184 s | |
| Unix time | counts UTC seconds | Computers; seconds since 1970, ignoring leap seconds | |
| NTP time | counts UTC seconds | Network Time Protocol; seconds since 1900 | |
| GPS week | from GPS time | Week number and seconds of the week, as receivers count them |
Why there are so many
Atomic time (TAI) simply counts seconds as measured by more than 400 atomic clocks worldwide. It never jumps. The Earth, however, does not rotate perfectly evenly: tides, the core and even large earthquakes change its speed slightly. Astronomers measure its actual rotation as UT1, which drifts away from atomic time by a fraction of a second per year.
UTC is the compromise used for civil time: it ticks with atomic seconds, but when it drifts too far from the Earth’s rotation (more than 0.9 seconds), a leap second is added. That is why UTC is now 37 seconds behind TAI. Navigation systems such as GPS cannot tolerate jumps, so they run continuously from the moment they started, which explains their fixed differences from TAI.
Leap seconds
A leap second is inserted as an extra second, 23:59:60 UTC, at the end of June or December. There have been 27 since 1972, all of them added; none has ever been removed. The last one was on 31 December 2016. Since then the Earth has rotated slightly faster, and a negative leap second has been discussed for the first time.
Leap seconds cause real problems for computers, which usually cannot represent 23:59:60. Some systems repeat a second, others freeze; past leap seconds have crashed servers and websites. Large companies such as Google, Meta and Amazon therefore smear leap seconds: they slow their clocks slightly over many hours instead of inserting a second. Never mix smearing and non-smearing NTP servers.
The end of leap seconds: in 2022 the General Conference on Weights and Measures decided to allow UTC and UT1 to drift further apart, by or before 2035. That effectively ends leap seconds for at least a century.
All leap seconds since 1972
When UTC started in its current form on 1 January 1972, TAI was already 10 seconds ahead. Each leap second added one more.
| # | Inserted at the end of | TAI − UTC afterwards |
|---|---|---|
| 1 | 30 June 1972 (23:59:60 UTC) | 11 s |
| 2 | 31 December 1972 (23:59:60 UTC) | 12 s |
| 3 | 31 December 1973 (23:59:60 UTC) | 13 s |
| 4 | 31 December 1974 (23:59:60 UTC) | 14 s |
| 5 | 31 December 1975 (23:59:60 UTC) | 15 s |
| 6 | 31 December 1976 (23:59:60 UTC) | 16 s |
| 7 | 31 December 1977 (23:59:60 UTC) | 17 s |
| 8 | 31 December 1978 (23:59:60 UTC) | 18 s |
| 9 | 31 December 1979 (23:59:60 UTC) | 19 s |
| 10 | 30 June 1981 (23:59:60 UTC) | 20 s |
| 11 | 30 June 1982 (23:59:60 UTC) | 21 s |
| 12 | 30 June 1983 (23:59:60 UTC) | 22 s |
| 13 | 30 June 1985 (23:59:60 UTC) | 23 s |
| 14 | 31 December 1987 (23:59:60 UTC) | 24 s |
| 15 | 31 December 1989 (23:59:60 UTC) | 25 s |
| 16 | 31 December 1990 (23:59:60 UTC) | 26 s |
| 17 | 30 June 1992 (23:59:60 UTC) | 27 s |
| 18 | 30 June 1993 (23:59:60 UTC) | 28 s |
| 19 | 30 June 1994 (23:59:60 UTC) | 29 s |
| 20 | 31 December 1995 (23:59:60 UTC) | 30 s |
| 21 | 30 June 1997 (23:59:60 UTC) | 31 s |
| 22 | 31 December 1998 (23:59:60 UTC) | 32 s |
| 23 | 31 December 2005 (23:59:60 UTC) | 33 s |
| 24 | 31 December 2008 (23:59:60 UTC) | 34 s |
| 25 | 30 June 2012 (23:59:60 UTC) | 35 s |
| 26 | 30 June 2015 (23:59:60 UTC) | 36 s |
| 27 | 31 December 2016 (23:59:60 UTC) | 37 s |
Rollover dates programmers should know
- 19 January 2038, 03:14:07 UTC: signed 32-bit Unix time overflows. Countdown and details on the Unix timestamp converter.
- 7 February 2036, 06:28:16 UTC: the 32-bit NTP seconds counter starts its second era. Correct implementations handle this; very old ones may not.
- GPS week rollovers: older GPS receivers count weeks in 10 bits and wrap every 1,024 weeks (the last time was on 6 April 2019; the next is in November 2038).
Sources: BIPM (TAI and UTC), IERS (leap seconds and UT1), and the published specifications of GPS, Galileo and BeiDou.