Julian Date Converter
Convert Gregorian calendar dates to Julian Day Numbers (JDN) and Modified Julian Dates (MJD). Convert JDN back to calendar date. Used in astronomy, military, and GPS systems.
Julian Day Number (JDN)
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Modified Julian Date (MJD) —
Day of Year (DOY) —
Extended More scenarios, charts & detailed breakdown ▾
Julian Day Number (JDN)
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Astronomical JD (with time) —
Modified Julian Date (MJD) —
Professional Full parameters & maximum detail ▾
Julian Dates
Julian Day Number (JDN) —
Astronomical Julian Date —
Modified Julian Date (MJD) —
Related Systems
Julian Calendar Date (pre-Gregorian) —
Day of Year (DOY) —
GPS Week Number —
Unix Timestamp —
How to Use This Calculator
- Enter a year, month, and day to get the Julian Day Number and Modified Julian Date.
- Use Julian Day to Gregorian tab to convert a JDN back to a calendar date.
- Click Today's Julian Date for quick reference (April 5, 2026).
- Switch to Professional for the full astronomical JD with time, GPS week, Unix timestamp, and Julian Calendar date.
Formula
JDN = D + ⌊(153m+2)/5⌋ + 365y + ⌊y/4⌋ − ⌊y/100⌋ + ⌊y/400⌋ − 32045
MJD = JDN − 2,400,000.5
Astronomical JD = JDN − 0.5 + fractional day
Example
Example: April 5, 2026 (Gregorian) → JDN = 2,461,671. MJD = 2,461,671 − 2,400,000.5 = 61,670.5. Day of Year = 95.
Frequently Asked Questions
- The Julian Day Number is a continuous count of days since the beginning of the Julian Period on January 1, 4713 BC. For example, April 5, 2026 = JDN 2,461,671. It is used in astronomy to compute intervals between dates.
- MJD = JDN − 2,400,000.5. It starts at midnight rather than noon, and uses smaller numbers. MJD is common in satellite and geodetic calculations.
- The Julian Calendar is the pre-Gregorian calendar system introduced by Julius Caesar. The Julian Day Number is an astronomical day count with no relation to the Julian Calendar — they only share a name.
- GPS time began January 6, 1980 (JDN 2,444,244). GPS Week = (JDN − 2,444,244) ÷ 7. GPS receivers use week number + seconds-of-week instead of calendar dates.
- DOY is the sequential day number within a year. January 1 = DOY 1, December 31 = DOY 365 (or 366 in a leap year). It is commonly used in meteorology and satellite imagery.