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Julian Date, MJD and sidereal time converter (with BJD and HJD)

Convert any moment between UTC, your local time, Julian Date, Modified Julian Date and Unix time, and see sidereal time, ΔT and light-time corrected BJD for a star.

Keep it one tap away for everyday use.

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  1. Pick a time
  2. Convert
  3. Add a star
  4. Copy result
Nothing is uploadedWorks offline once open

Which moment?

Results live

Heliocentric and barycentric Julian Date (for timing a star)

Light from a star reaches the Sun or the Solar System's centre up to 8.3 minutes earlier or later than it reaches Earth. Enter the star's position (J2000) to correct for it.

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What this converter does

Astronomers use several clocks at once. Observing logs and variable-star reports use the Julian Date (JD), a running count of days. Catalogues and satellites often use the Modified Julian Date (MJD), which is the JD minus 2,400,000.5, so the numbers are shorter and each day starts at midnight. Telescope mounts need sidereal time, which tracks the stars rather than the Sun. Timing an eclipsing binary or an exoplanet transit needs the barycentric Julian Date (BJD), which removes the minutes that light takes to cross Earth’s orbit. This page shows all of them for one moment, side by side.

How to use it

  1. Set your place at the top. Local sidereal time and your local clock depend on your longitude and time zone. Pick a city, type your latitude and longitude, or press Use my location. Your location stays on your device.
  2. Choose the moment. Now gives a live clock that updates every second. Date and time reads your local time (tick This is UTC to type UTC instead). You can also paste a JD, MJD or Unix time to convert it back.
  3. Read the results. Every value updates as you type.
  4. Open Heliocentric and barycentric Julian Date and type a star’s J2000 position, or pick an object such as M13, to get the BJD (TDB) and HJD for that star.
  5. Download the results as an image, PDF or Word file for your notes.

What each result means

  • Julian Date (UT): days since noon on 1 January 4713 BC, on the UTC timescale.
  • Julian Date (TT): the same count on Terrestrial Time, the smooth timescale that ephemerides use. It runs about 69 seconds ahead of UTC, and that gap is shown as ΔT.
  • Greenwich and local sidereal time: the right ascension that is crossing the meridian right now. An object is highest in the sky when the local sidereal time equals its RA.
  • BJD (TDB): the time the light would have reached the Solar System’s centre of mass. This is the standard for exoplanet and pulsar timing.
  • HJD: the older correction to the Sun’s centre. It is still used in many variable-star papers.

Worked example

Suppose you observe Algol from Toronto at 22:00 local time on 7 November 2026. That is 03:00 UTC on 8 November, JD 2461352.625. Type Algol’s position (03h 08m 10s, +40° 57′ 20″) and the page shows a light-time correction of a few minutes. Add that to the JD (TT) and you get the BJD to report.

Questions

How accurate is it? Sidereal time and Julian Dates are exact to the second. The BJD correction uses the full planetary positions from astronomy-engine and matches professional tools to well under a second. That is plenty for amateur timing work, but check against a research tool for millisecond pulsar work.

Does it need the internet? No. Once the page has loaded, it keeps working offline.

Last updated 2026-11-07.

Change log for Astronomy time converter

  1. Tool #numbers, the name's meaning, and the MIT License Details on GitHub
  2. Ten astronomy tools on the Science shelf, with PNG, PDF and Word downloads Details on GitHub

Every change to fibub

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