Developer tools
Time standards converter
See one instant across every way of counting time - UTC, Unix, TAI, GPS, TT, Julian Date, and more. Pick a moment, or type a Julian Date, MJD, or Unix timestamp to set it. Everything runs in your browser - nothing is uploaded.
Pick a moment
Local civil time: -
Or set it from a value
Time standards
- UTC (ISO 8601)
- -
- Local civil time
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- Unix time (seconds)
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- Unix time (milliseconds)
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- TAI
- -
- GPS time
- -
- TT
- -
- Julian Date (JD)
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- Modified Julian Date (MJD)
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- Day of year
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- ISO-8601 week date
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- GMST
- -
- TAI - UTC offset
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Coordinated Universal Time, the global civil time standard.
The same instant in your browser's time zone.
Seconds since 1970-01-01 UTC, ignoring leap seconds.
The same count in milliseconds, as JavaScript reports it.
International Atomic Time = UTC + leap seconds (-).
Used by GPS satellites; -.
Terrestrial Time = UTC + leap + 32.184s (-), used in astronomy.
Days since noon UTC, 1 January 4713 BC.
JD shifted to start at midnight, zeroed at 1858-11-17.
Ordinal day number within the UTC year (1 to 366).
Week year, week number, and weekday (1 = Monday).
Greenwich Mean Sidereal Time, the sky's clock (low precision).
Current TAI - UTC offset (whole leap seconds since 1972).
How it works
A time zone just shifts the same instant by a fixed offset, but a time standard changes how time itself is counted. This converter takes one moment and shows it in every common standard at once: UTC and your local civil time, Unix time in seconds and milliseconds, the atomic family (TAI, GPS time, and TT), the astronomical Julian Date and Modified Julian Date, the day of year and ISO-8601 week date, and Greenwich Mean Sidereal Time. Pick a moment with the UTC picker, jump to the live "now", or work backwards by typing a Julian Date, MJD, or Unix timestamp to set the instant.
The atomic standards differ from UTC only by a whole number of leap seconds, which are added every so often to keep UTC in step with the Earth's slightly irregular rotation. TAI (International Atomic Time) runs ahead of UTC by the current leap-second count, GPS time is fixed to TAI minus 19 seconds, and TT (Terrestrial Time) is TAI plus 32.184 seconds. The tool carries a bundled leap-second table from 1972 onward, so the offsets are accurate for any date in that range, and it shows the exact TAI minus UTC value in effect for the moment you chose.
Example. At 2000-01-01T12:00:00Z the Julian Date is exactly 2451545.0 (the J2000.0 epoch astronomers use as a reference), the Modified Julian Date is 51544.5, and the leap-second offset of the day was 32 seconds, so TAI reads 2000-01-01T12:00:32. Today the offset is 37 seconds, which makes GPS time 18 seconds ahead of UTC. Type a Unix timestamp into the reverse panel and every one of these updates to match.
FAQ
What is the difference between a time standard and a time zone?
A time zone is just an offset: Tokyo and London show different clock times for the same instant, but they agree on what that instant is. A time standard is a different system for counting time altogether. UTC, Unix time, TAI, GPS time, and Julian Date all describe the same physical moment, yet they number it differently - some count leap seconds, some do not, and some start their count from a completely different epoch. This tool converts between the standards; a separate time zone converter handles the offsets.
What are TAI, GPS time, and TT?
They are members of the atomic-time family, all tied together by fixed offsets. TAI (International Atomic Time) is the weighted average of hundreds of atomic clocks and runs ahead of UTC by the current whole number of leap seconds. GPS time is what GPS satellites broadcast; it was set equal to UTC at its 1980 start and has not taken leap seconds since, so it is always TAI minus 19 seconds. TT (Terrestrial Time) is the theoretical standard used for astronomical ephemerides and equals TAI plus 32.184 seconds.
What are Julian Date and Modified Julian Date?
Julian Date (JD) is a continuous day count used in astronomy: it is the number of days, including a fraction, since noon UTC on 1 January 4713 BC. Because that makes JD a large seven-digit number, the Modified Julian Date (MJD) shifts the origin to 1858-11-17 and starts each day at midnight instead of noon, so MJD = JD minus 2400000.5. Both are handy for arithmetic on dates far apart, since you can just subtract them to get the number of days between two instants.
How accurate is the leap-second offset, and is anything uploaded?
The converter bundles the full leap-second table from 1972 to the latest entry (37 seconds, from 2017), so the TAI, GPS, and TT offsets are exact for any date in that range. Future leap seconds cannot be known in advance, and dates before 1972 used fractional adjustments rather than whole seconds, so those are flagged as approximate. Everything is plain arithmetic running in your browser - nothing you enter is uploaded, logged, or stored, and the tool works offline once loaded.