GPS Time, TAI and Leap Second Converter
Convert UTC to GPS week and time of week, TAI and the GPS−UTC offset, with every leap second since 1972 listed.
TAI − UTC has been 37 s since 2017-01-01 · no later change in this table
Every leap second so far
| In effect from (UTC) | TAI − UTC | |
|---|---|---|
| 1972-01-01 | 10 s | the offset UTC started with, not an inserted second |
| 1972-07-01 | 11 s | one second inserted at the end of the day before |
| 1973-01-01 | 12 s | one second inserted at the end of the day before |
| 1974-01-01 | 13 s | one second inserted at the end of the day before |
| 1975-01-01 | 14 s | one second inserted at the end of the day before |
| 1976-01-01 | 15 s | one second inserted at the end of the day before |
| 1977-01-01 | 16 s | one second inserted at the end of the day before |
| 1978-01-01 | 17 s | one second inserted at the end of the day before |
| 1979-01-01 | 18 s | one second inserted at the end of the day before |
| 1980-01-01 | 19 s | one second inserted at the end of the day before |
| 1981-07-01 | 20 s | one second inserted at the end of the day before |
| 1982-07-01 | 21 s | one second inserted at the end of the day before |
| 1983-07-01 | 22 s | one second inserted at the end of the day before |
| 1985-07-01 | 23 s | one second inserted at the end of the day before |
| 1988-01-01 | 24 s | one second inserted at the end of the day before |
| 1990-01-01 | 25 s | one second inserted at the end of the day before |
| 1991-01-01 | 26 s | one second inserted at the end of the day before |
| 1992-07-01 | 27 s | one second inserted at the end of the day before |
| 1993-07-01 | 28 s | one second inserted at the end of the day before |
| 1994-07-01 | 29 s | one second inserted at the end of the day before |
| 1996-01-01 | 30 s | one second inserted at the end of the day before |
| 1997-07-01 | 31 s | one second inserted at the end of the day before |
| 1999-01-01 | 32 s | one second inserted at the end of the day before |
| 2006-01-01 | 33 s | one second inserted at the end of the day before |
| 2009-01-01 | 34 s | one second inserted at the end of the day before |
| 2012-07-01 | 35 s | one second inserted at the end of the day before |
| 2015-07-01 | 36 s | one second inserted at the end of the day before |
| 2017-01-01 | 37 s | one second inserted at the end of the day before |
Leap seconds come from the IERS table the operating system ships; GPS − UTC is TAI − UTC minus the fixed 19 s.
How to use
- Enter a UTC date and time to get its TAI and GPS equivalents, or switch to the second mode and enter a GPS week and time of week to go back.
- Read TAI − UTC and GPS − UTC beside the result: they are what the conversion applied, not decoration.
- Click any value to copy it.
- Check the rollover era when a receiver reports a date about nineteen years out — that is the ten-bit week counter wrapping.
About this tool
ShowHide
Convert an instant between UTC, TAI and GPS time, in either direction, and see the GPS week number, day of week, time of week and seconds since the GPS epoch beside the offsets that produced them. The three scales differ for one reason: TAI counts SI seconds and never stops, UTC inserts a leap second whenever Earth's rotation has drifted far enough, and GPS was set to TAI minus exactly 19 seconds in 1980 and then left alone. So TAI − UTC grows in steps — 10 seconds when the system started in 1972, 37 since the last insertion at the start of 2017 — while GPS − UTC is simply that figure minus 19, which is why GPS has run 18 seconds ahead of UTC for years. Getting this wrong shifts a timestamp by nearly twenty seconds, which is the difference between a fix landing on the right road and the next one. Two traps live in the week number. It is broadcast in ten bits, so it wraps every 1024 weeks — in 1999, in 2019, and next in 2038 — and a receiver that ignores the rollover reports a date nineteen years and eight months early. And the week begins at midnight between Saturday and Sunday, so day 0 is Sunday, not Monday, which is the opposite of the ISO convention most date libraries use. Converting the other way is not a clean inverse either: GPS − UTC depends on the instant you are solving for, so a GPS reading close to a midnight where a leap second was inserted has to be resolved rather than computed in one step.
Frequently asked questions
Why is GPS time 18 seconds ahead of UTC?
GPS was aligned to UTC at its epoch, 1980-01-06, when TAI − UTC was 19 seconds, and GPS was fixed at TAI minus 19 seconds for good. UTC has taken 18 more leap seconds since then, and GPS took none, so the gap is now 18 seconds. It will grow by one more second whenever the next leap second is inserted.
What is the GPS week rollover?
The week number in the broadcast navigation message is ten bits, so it counts to 1023 and wraps. It has wrapped twice, in August 1999 and April 2019, and will again in November 2038. A receiver that does not account for it reports a date 1024 weeks — about nineteen years and eight months — too early. Newer message formats carry thirteen bits, which pushes the problem out to 2137.
Is day 0 of the GPS week Sunday or Monday?
Sunday. The GPS week starts at midnight between Saturday and Sunday, so time of week 0 is Sunday 00:00:00 and day of week runs 0–6 from Sunday. ISO 8601 numbers weekdays 1–7 from Monday, so a naive conversion is off by one and wraps at the weekend.
Why can UTC before 1972 not be converted?
Leap seconds only began in 1972. Before that UTC was kept in step with Earth by changing the length of the second itself and by fractional offsets — so-called rubber-band time — and there is no whole-second TAI − UTC to apply. The table this tool uses starts at 1972-01-01 with an offset of 10 seconds.
Will there be more leap seconds?
Fewer, and then none: in 2022 the General Conference on Weights and Measures resolved to stop inserting them by 2035. None has been added since the end of 2016, and Earth has lately been running slightly fast, which would call for a negative leap second — something never yet done and widely expected to break software that assumes seconds only get added.
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