Julian Day Converter
Convert Gregorian calendar dates to Julian Day Numbers (JDN).
Date Add / Subtract Calculator
About Julian Day Converter
The Julian Day Number (JDN) & Modified Julian Date (MJD) Converter translates Gregorian/Julian calendar dates and UTC times to astronomical Julian Date ($JD$), Modified Julian Date ($MJD = JD - 2400000.5$), and Heliocentric / Truncated Julian Day representations.
How to Use Julian Day Converter
Step 1
Enter Calendar Date and Time in UTC (or paste a Julian Day number).
Step 2
Review computed JD, MJD, and astronomical epoch offsets.
Step 3
Click "Copy Julian Date (JD)".
Practical Use Cases for Julian Day Converter
Astronomical Ephemeris & Celestial Body Orbit Tracking
Convert observation timestamps to continuous Julian Dates for orbital mechanics calculations, planetarium software, and satellite tracking.
Satellite Doppler Tracking & Aerospace Telemetry Epochs (NASA / ESA)
Compute Modified Julian Dates ($MJD$) for GPS orbital epochs, NORAD two-line element sets (TLE), and spacecraft deep-space navigation.
Input & Output Examples
Converting January 1, 2000 12:00:00 UTC (J2000.0 Epoch)
Date & Time: `2000-01-01 12:00:00 UTC`
Julian Date (JD): `2451545.0` | Modified Julian Date (MJD): `51544.5` | J2000.0 Offset: `0.0 days` | Unix Epoch: `946728000`
Key Features & Performance
- ✓High-Precision Continuous Time: calculates JD with 6 decimal places of precision ($< 0.1\text{ second}$).
- ✓Supported Epochs: Julian Date ($JD$), Modified Julian Date ($MJD$), Truncated Julian Day ($TJD$), Chronological JD ($CJD$), and J2000.0 delta.
- ✓Bi-Directional Converter: paste Julian Date number to instantly reconstruct calendar date and UTC time.
- ✓100% Client-Side astronomical engine.
- ✓1-Click Copy astronomical Julian day numbers.
Key Terminology & Definitions
Julian Date ($JD$)
The continuous count of days and fractions since noon Universal Time on January 1, 4713 BC (Julian proleptic calendar), widely used by astronomers to eliminate calendar complexities.
Modified Julian Date ($MJD$)
A simplified astronomical date system defined as $MJD = JD - 2400000.5$, starting at midnight on November 17, 1858, reducing number sizes.