Julian Date Converter
Convert Julian dates
Frequently Asked Questions
What is a Julian date?
A Julian date (JD) is a continuous count of days since January 1, 4713 BC. For example, January 1, 2025 at noon is JD 2460677.0. It eliminates calendar complexities and is widely used in astronomy and historical chronology.
How do I convert a calendar date to Julian date?
Enter any Gregorian calendar date and the calculator returns the Julian Day Number. The conversion involves accounting for leap years, month lengths, and the Gregorian calendar reform. JD values increase by 1 each day at noon UTC.
What is the difference between Julian date and Julian day number?
Julian Day Number (JDN) is the integer part — the whole day count. Julian Date (JD) includes the fractional day. JD 2460677.75 means JDN 2460677 at 6:00 PM UTC (0.75 of a day past noon).
Why do astronomers use Julian dates?
Julian dates provide a uniform time scale without calendar irregularities (no months, no leap year rules). Calculating the time between two events is simple subtraction. JD 2460700 - JD 2460677 = exactly 23 days.
What is Modified Julian Date (MJD)?
MJD = JD - 2400000.5. It starts at midnight (not noon) and uses smaller numbers. January 1, 2025 midnight = MJD 60676. MJD is used in satellite tracking, telecommunications, and modern astronomical applications.