Julian Date Converter: JD, MJD and Ordinal Date

How to use the julian date converter

1. Choose the conversion direction: Gregorian to Julian Date or Julian Date to Gregorian.
2. For gregorian to julian date, enter the calendar date as YYYY-MM-DD and an optional UT time as HH:MM.
3. For the reverse, enter the Julian Date number (for example 2451545.0).
4. Read the results instantly: JD, Modified Julian Date and ordinal day of year appear below.

How the Julian Date is calculated

The Julian Date counts days continuously from noon Universal Time on January 1, 4713 BC in the proleptic Julian calendar. The standard algorithm converts a Gregorian year Y, month M and day D (with fractional day) into a single number.

First, adjust January and February by treating them as months 13 and 14 of the previous year. Then compute:

A = floor(Y / 100)
B = 2 - A + floor(A / 4)
JD = floor(365.25 * (Y + 4716)) + floor(30.6001 * (M + 1)) + D + B - 1524.5

For the J2000.0 epoch (2000-01-01 at 12:00 UT): Y becomes 1999, M becomes 14, D is 1.5. A = 19, B = -13. The formula yields 2451545.0, which is the reference point for modern astronomy.

Reference values for common epochs

This table lists well-known moments and their corresponding Julian Date values. Use them to verify your own calculations or to calibrate a julian day calculator.

Julian Date versus day of year

The term "Julian date" is used in two different ways, which causes frequent confusion. In astronomy, it means the continuous day count described above. In logistics, manufacturing and food packaging, it often refers to the ordinal day of the year: January 1 is day 1, February 1 is day 32, and December 31 is day 365 (or 366 in leap years).

This julian date converter provides both values so you can use whichever definition your work requires. The ordinal date is labelled "Day of year" in the output to keep the distinction clear.

Modified Julian Date

The Modified Julian Date (MJD) simplifies record keeping by subtracting 2,400,000.5 from the full Julian Date. Its epoch is midnight on November 17, 1858. Because MJD values are smaller, they fit more easily into databases and spreadsheets while preserving sub-day precision.

MJD = JD - 2400000.5

For example, J2000.0 at JD 2451545.0 corresponds to MJD 51544.5. Many satellite tracking systems and observatory logs prefer MJD over the full JD.

Common mistakes to avoid

When performing a gregorian to julian date conversion, several pitfalls produce wrong results:

• Ignoring the noon start. Julian Dates begin at 12:00 UT, not midnight. A date entered as 2000-01-01 00:00 UT gives JD 2451544.5, not 2451545.0.
• Local time instead of UT. Always convert local times to Universal Time before entering them. This tool uses UTC arithmetic internally, but the input must already represent UT.
• Invalid calendar dates. February 29 only exists in leap years. The converter validates every date against the Gregorian calendar rules and rejects impossible combinations.
• Confusing Julian calendar with Julian Date. The Julian calendar is the older civil calendar replaced by the Gregorian reform in 1582. The Julian Date is a continuous astronomical count unrelated to that calendar system.

This tool uses the proleptic Gregorian calendar for every date, including those before the 1582 reform, in both directions. Historical records written in the Julian calendar need converting to Gregorian first.

If you need to shift a date forward or backward by a fixed span rather than convert it, a tool like add days to date may be more appropriate.

Worked examples