Julian Date Converter

The Julian Date Converter turns any calendar date into its Julian Date (JD) and Julian Day Number (JDN) — and converts straight back the other way. It also gives you the Modified Julian Date (MJD), days since the J2000.0 epoch, the Unix timestamp, the day of the year, the weekday, and the 5-digit ordinal "Julian date" used in industry — all from a single conversion, with a visual time-scale and a step-by-step formula.

What Is a Julian Date?

A Julian Date is a continuous count of days, including the fraction of a day, measured from noon Universal Time on January 1, 4713 BC (in the proleptic Julian calendar). Because it ignores months, varying month lengths, leap years and calendar reforms, it makes date arithmetic trivial: the number of days between two events is simply one Julian Date subtracted from another. Astronomers, satellite operators and historians rely on it for exactly this reason.

Julian Date vs. Julian Day Number

These two terms are often mixed up:

• Julian Day Number (JDN) — the whole-number day count, with no time of day.
• Julian Date (JD) — the JDN plus the fraction of the day for the time. Since the day begins at noon, midnight carries a .5 fraction.

The Julian Date Formula

For a date in the Gregorian calendar, with the time folded into a fractional day D, the Julian Date is computed in three short steps.

For the Julian calendar, the correction term is simply B = 0. To go the other way — from a Julian Date back to a calendar date — the tool applies Meeus' inverse algorithm, which automatically returns Gregorian dates on or after October 15, 1582 and Julian dates before it.

Related Julian Date Variants

The 5-Digit "Julian Date" Is Different

In manufacturing, food packaging, military logistics and mainframe systems, a "Julian date" usually means a 5-digit ordinal date such as 26180, made of the last two digits of the year followed by the day of the year (day 180 of 2026). It has nothing to do with the astronomical Julian Day count. To avoid any confusion, this converter shows both the true Julian Date and the ordinal date for every result.

Worked Example

Take noon UT on January 1, 2000 — the famous J2000.0 epoch:

• Month is January, so set Y = 1999, M = 13.
• Gregorian correction: A = 19, B = −13.
• JD = 730196 + 428 + 1.5 − 13 − 1524.5 = 2451545.0, with JDN 2451545.

How to Use This Calculator

1. Choose a direction: "Date → Julian" to convert a calendar date, or "Julian → Date" to convert a JD, MJD or Unix value.
2. Enter your values: For a date, fill in the year, month, day and time and pick the Gregorian or Julian calendar. For a Julian input, type the number and select its type.
3. Click Convert: The astronomical conversion runs instantly.
4. Read the results: See the Julian Date and every related format, copy any value with one click, locate your date on the time-scale and 24-hour dial, and follow the step-by-step formula.

Frequently Asked Questions

What is a Julian Date?

A Julian Date (JD) is a continuous count of days and fractions of a day since noon Universal Time on January 1, 4713 BC in the proleptic Julian calendar. Astronomers use it because it removes months, leap years and calendar reforms from date arithmetic, so the difference between two events is simply a subtraction.

What is the difference between Julian Date and Julian Day Number?

The Julian Day Number (JDN) is the integer day count, while the Julian Date (JD) includes the fraction of the day for the time. For example, noon on January 1, 2000 has JDN 2451545 and JD 2451545.0, while midnight that morning has JD 2451544.5.

What is the Modified Julian Date (MJD)?

The Modified Julian Date is MJD = JD − 2400000.5. It uses smaller numbers and, unlike the Julian Date, its day starts at midnight rather than noon, which makes it convenient for satellite tracking and many computing applications.

Why does the Julian Day start at noon?

The Julian Day begins at noon Universal Time so that a single night of astronomical observation falls within one Julian Day rather than being split across a midnight date change. This is why a civil midnight carries a 0.5 day fraction.

Is the 5-digit Julian date the same thing?

No. The 5-digit ordinal date used in manufacturing, food packaging and mainframes, such as 26180, is the last two digits of the year followed by the day of the year. It is sometimes called a Julian date but is unrelated to the astronomical Julian Day count. This converter shows both so you are never confused.

Which calendar should I choose?

Use Gregorian for modern dates and most purposes. Choose Julian only for historical dates before the 1582 reform when your source explicitly uses the old-style Julian calendar. When converting a Julian Date back to a date, this tool automatically returns Gregorian for dates on or after October 15, 1582 and Julian before it.

Additional Resources

• Julian Day - Wikipedia
• Astronomical Epoch (J2000.0) - Wikipedia