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Julian Date Converter

Time & Speed

Convert a Gregorian calendar date to its Julian Day Number (JDN) and Modified Julian Date (MJD), or convert a Julian Day Number back to a calendar date.

Reviewed by the thecalcu.com team · Last updated July 27, 2026

Julian Day Number (JDN)
24,61,257
Modified Julian Date (MJD)
61,256.5
Day of Week
Tuesday

What is a Julian Date?

A Julian Date Converter translates between ordinary calendar dates and the Julian Day Number (JDN) system, a continuous day count used by astronomers, historians, and some scientific software instead of the year-month-day format most people use daily. The Julian Day Number counts every day sequentially from a fixed starting point (noon UTC, January 1, 4713 BCE), which makes it possible to calculate the exact number of days between any two dates with a single subtraction, without worrying about differing month lengths or leap years.

This converter also computes the Modified Julian Date (MJD), a more compact variant that shifts the reference point to 1858 and aligns the day boundary with midnight instead of noon, the format most commonly used in modern satellite and scientific data. It's a companion to the Unix Timestamp Converter, which handles the computing world's epoch-based timestamp system rather than the astronomical Julian Day system.

Why Use a Julian Date Converter?

Astronomy software, satellite tracking systems, and some older scientific and historical datasets record dates purely as Julian Day Numbers, which aren't human-readable without conversion. Manually working out what "JDN 2461234" means in calendar terms, or converting a calendar date into the JDN a piece of software expects, requires an algorithm most people don't have memorised.

Two concrete situations this solves: converting an astronomical observation's reported JDN into a readable calendar date to cross-reference against other records, and calculating a modern date's Julian Day Number to feed into an ephemeris calculator or scientific script that expects that input format. Both conversions happen instantly here without needing to implement the underlying calendar algorithm yourself.

Who Should Use This Converter?

  • Astronomy students and hobbyists converting between calendar dates and the Julian Day Numbers used in ephemeris tables, star charts, and planetarium software.
  • Researchers working with historical or scientific datasets that record dates as Julian Day Numbers or Modified Julian Dates rather than calendar dates.
  • Software developers integrating with astronomical APIs, satellite tracking systems, or scientific libraries that require JDN or MJD as input.
  • Genealogists and historians cross-referencing old records that use Julian Day-based dating systems against modern calendar dates.
  • Students studying orbital mechanics or celestial navigation, where day-difference calculations rely on the continuous JDN count rather than calendar arithmetic.

What Insights Does the Julian Date Converter Give You?

When converting a calendar date to Julian Day Number, the tool shows the Julian Day Number itself (a running integer), the Modified Julian Date (JDN minus 2,400,000.5, aligned to midnight), and the day of the week for quick verification. When converting the other direction, entering a Julian Day Number reveals the exact calendar date it corresponds to, along with the same day-of-week and MJD cross-reference.

The day-of-week output doubles as a sanity check: if you already know what day of the week a historical or astronomical event fell on, you can confirm the converter's date output matches before using it in further calculations.

How to use this Julian Date calculator

  1. Choose a direction using the tabs: Date → Julian Day to convert a calendar date, or Julian Day → Date to convert a JDN back to a calendar date.
  2. In Date → Julian Day mode, select your date using the Calendar Date field.
  3. Read the Julian Day Number result, along with the Modified Julian Date and day of the week shown below it.
  4. In Julian Day → Date mode, type a Julian Day Number into the Julian Day Number field, such as 2461234.
  5. Read the resulting Calendar Date, day of the week, and Modified Julian Date.
  6. Use the copy or export options to save the result for reference in astronomical software or research notes.

Formula & Methodology

This converter uses the Fliegel & Van Flandern algorithm to convert a proleptic Gregorian calendar date to a Julian Day Number:

a = ⌊(14 − month) ÷ 12⌋, y = year + 4800 − a, m = month + 12a − 3

JDN = day + ⌊(153m + 2) ÷ 5⌋ + 365y + ⌊y ÷ 4⌋ − ⌊y ÷ 100⌋ + ⌊y ÷ 400⌋ − 32045

The reverse conversion (JDN to calendar date) uses the algebraic inverse of this formula. Modified Julian Date is calculated simply as MJD = JDN − 2,400,000.5.

Worked example: Converting January 1, 2000 gives a = ⌊(14−1)÷12⌋ = 1, y = 2000+4800−1 = 6799, m = 1+12−3 = 10. Plugging into the formula: JDN = 1 + ⌊(153×10+2)÷5⌋ + 365×6799 + ⌊6799÷4⌋ − ⌊6799÷100⌋ + ⌊6799÷400⌋ − 32045 = 2,451,545, the well-known Julian Day Number for the start of the year 2000 (noon UTC).

Frequently Asked Questions

The Julian Day Number (JDN) is a continuous count of days since noon UTC on January 1, 4713 BCE (in the proleptic Julian calendar), used by astronomers and some scientific fields to avoid the irregularities of calendar months and years. Because it's a single running integer rather than a year-month-day combination, it makes calculating the number of days between two dates a simple subtraction.
Modified Julian Date (MJD) is defined as JDN minus 2,400,000.5, which shifts the reference point to midnight on November 17, 1858, and produces a smaller, more convenient number for modern dates. The 0.5 offset also moves the day boundary from noon (used by JDN) to midnight, matching how calendar dates are normally understood.
The standard algorithm (Fliegel & Van Flandern) computes JDN from the proleptic Gregorian year, month, and day using a series of integer floor-division steps, correctly handling leap years and the Gregorian calendar's 400-year leap cycle. This converter implements that exact algorithm, so results match the values used in astronomical software and reference almanacs.
Switch to the 'Date → Julian Day' tab, pick your date using the calendar date field, and the Julian Day Number, Modified Julian Date, and day of the week appear instantly. There's no need to press a convert button, the result updates as soon as you change the date.
Switch to the 'Julian Day → Date' tab, type the Julian Day Number into the input field, and the tool immediately shows the corresponding Gregorian calendar date, day of the week, and Modified Julian Date. This is useful when working with astronomical datasets or older scientific records that report dates only as JDN values.
Julian Day Number avoids the complications of varying month lengths, leap years, and calendar reforms, making it far easier to calculate elapsed time between two events, such as measuring the interval between two eclipses or satellite observations. A single subtraction between two JDN values gives the exact number of days, something that requires much more careful logic with month/day/year dates.
This converter calculates the day of the week directly from the Julian Day Number using its remainder when divided by 7, since JDN 0 corresponds to a Monday. Both conversion directions display the day of the week alongside the date and JDN/MJD values.
No, despite the similar name, the Julian Day Number is a continuous day-counting system independent of any calendar, while the Julian calendar is the predecessor to the modern Gregorian calendar, used in Europe before 1582. This converter works entirely in the proleptic Gregorian calendar, projected backward for dates before Gregorian adoption, which is the standard convention for JDN calculations.
A Julian Day Number, by definition, begins at noon UTC, so JDN 2451545 refers to the period from 12:00 UTC on January 1, 2000 to 12:00 UTC the following day. This converter calculates JDN at the noon boundary for the selected calendar date, which is the standard astronomical convention.
A Unix timestamp counts seconds since midnight UTC on January 1, 1970, and is used almost universally in computing and web development, while Julian Day Number counts whole days since 4713 BCE and is used mainly in astronomy and scientific calendars. Use the [Unix Timestamp Converter](/unix-timestamp-converter/) for software timestamps and this tool for astronomical or scientific date calculations.
Also known as
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