Julian date, MJD and GPS week converter
Convert between a UTC calendar date and time, Julian date, Modified Julian date, GPS week and seconds of week, and Unix time, all at once.
Runs entirely in your browser
Julian dates here are based on UTC (not Terrestrial Time), as used in everyday and navigation work. GPS time is computed from UTC with the 18 leap seconds in effect since 2017, so a future leap second would change it. Not for navigation.
How to convert a Julian date
- Choose which value you have: a calendar date and time in UTC, a Julian date, a Modified Julian date, a GPS week and seconds, or a Unix time.
- Type it in. The page converts it to every other form straight away.
- Copy the value you need, such as the Julian date or the GPS week.
The formulas
A calendar date is converted to a Julian date with the algorithm in Jean Meeus, Astronomical Algorithms (2nd ed., 1998), chapter 7, and the page uses the equivalent Unix-time relation JD = Unix seconds / 86,400 + 2,440,587.5, which holds for the proleptic Gregorian calendar of your browser. The Modified Julian date is MJD = JD − 2,400,000.5 (IAU, 1973). The J2000.0 epoch is JD 2,451,545.0.
GPS time is a continuous time scale that began at 1980-01-06 00:00:00 UTC (Interface Specification IS-GPS-200). The GPS week is floor(seconds since that epoch / 604,800), the seconds of week is the remainder, and the broadcast 10-bit week is the week modulo 1,024. GPS time equals UTC plus the number of leap seconds inserted since 1980: 18 since 2017-01-01, from the table of 18 insertions published by the IERS (Bulletin C). The tool applies the count valid at the instant in question.
Leap seconds are announced by the IERS only months ahead, so GPS times for dates in the future assume no new leap second. Not for navigation.
Questions
What is a Julian date?
The Julian date (JD) is a continuous count of days since noon UT on 1 January 4713 BC in the Julian calendar. It is used in astronomy because it avoids months and years. For example, 2000-01-01 12:00 UTC is JD 2451545.0, the J2000.0 epoch.
What is the Modified Julian date?
MJD = JD − 2400000.5. It starts at midnight instead of noon and has fewer digits: 2000-01-01 00:00 is MJD 51544.0. Space agencies and satellite operators use it.
How do GPS weeks work?
GPS time started at 00:00 UTC on 6 January 1980 and counts weeks and seconds of the week from there, without leap seconds. The week number used to be broadcast in 10 bits, so it rolls over every 1,024 weeks (about 19.6 years): on 22 August 1999 and again on 6 April 2019. The next rollover is on 20 November 2038. This tool shows both the full week count and the 10-bit value.
Why is GPS time ahead of UTC?
GPS time does not insert leap seconds, while UTC has added 18 of them since 1980. GPS time is therefore 18 seconds ahead of UTC since 1 January 2017. The tool uses the leap seconds that have been announced so far.
Is the Julian date the same as the Julian calendar?
No. The Julian calendar is an old calendar with a leap year every four years. The Julian date is a day count. The date shown here is in the modern (proleptic Gregorian) calendar, as in your browser.
How precise is the conversion?
A double-precision Julian date has about 0.1 millisecond resolution today. The tool shows six decimals of a day, which is about 0.09 seconds.