Julian Date Converter
Convert between Gregorian calendar dates and Julian Day Numbers (JD). Get the Modified Julian Date (MJD), Julian Day of Year, day of the week, and more. Essential for astronomers, historians, and programmers.
What Is a Julian Date?
The Julian Date (JD) is a continuous count of days since noon Universal Time (UT) on January 1, 4713 BCE (proleptic Julian calendar). The system was introduced by Joseph Scaliger in 1583 and is widely used in astronomy for time calculations because it avoids the complexities of calendars with months and leap years. The Julian Day Number is the integer part of the Julian Date.
Two common variants are:
- Julian Date (JD): The full count of days since the epoch, including the fractional day.
- Modified Julian Date (MJD): Defined as MJD = JD − 2400000.5, making the epoch midnight on November 17, 1858. MJD is more compact for modern dates.
The Julian Day of Year is the day number within the current year (1 to 365 or 366 in a leap year).
How Does the Julian Date Converter Work?
The calculator uses the Fliegel-Van Flandern algorithm for Gregorian-to-Julian conversion:
Gregorian to Julian Date:
a = (14 − month) / 12
y = year + 4800 − a
m = month + 12a − 3
JD = day + floor((153m + 2) / 5) + 365y + floor(y/4) − floor(y/100) + floor(y/400) − 32045
Julian to Gregorian Date:
a = JD + 32044
b = (4a + 3) / 146097
c = a − (146097b) / 4
d = (4c + 3) / 1461
e = c − (1461d) / 4
m = (5e + 2) / 153
day = e − (153m + 2) / 5 + 1
month = m + 3 − 12 × (m / 10)
year = 100b + d − 4800 + (m / 10)
The fractional part of the JD represents the time of day, where 0.0 = midnight, 0.25 = 06:00, 0.5 = noon, and 0.75 = 18:00.
Why Use This Julian Date Converter?
- Bidirectional Conversion: Convert calendar dates to JD and JD back to calendar dates.
- Multiple Formats: Get JD, MJD, and Julian Day of Year.
- Day of Week: Instantly find the day of the week for any date.
- Precise Time: Include hours, minutes, and seconds for accurate JD calculations.
- Free & Private: No registration, no data storage.
❓ Julian Date Converter FAQ
What is a Julian Date (JD)?
The Julian Date is a continuous count of days since noon UT on January 1, 4713 BCE. It's the standard time system used in astronomy for calculations. The integer part is the Julian Day Number.
What is the Modified Julian Date (MJD)?
The Modified Julian Date is defined as MJD = JD − 2400000.5. It starts at midnight on November 17, 1858. MJD is more compact for modern dates and is commonly used in satellite navigation and astronomy.
What is the Julian Day of Year?
The Julian Day of Year is the number of days elapsed since January 1 of the current year. January 1 is day 1, and December 31 is day 365 (or 366 in a leap year).
What is the difference between JD and MJD?
JD is the full count since January 1, 4713 BCE, and is a large number (~2.46 million). MJD = JD − 2400000.5, making it more compact (~60,000 for modern dates). MJD also shifts the epoch from noon to midnight.
What is the epoch of the Julian Date?
The Julian Date epoch is noon (12:00) UT on January 1, 4713 BCE in the proleptic Julian calendar. This corresponds to November 24, 4714 BCE in the Gregorian calendar.
How do I convert a date to Julian Date?
Enter the year, month, day, hour, minute, and second. The calculator uses the Fliegel-Van Flandern algorithm to compute the JD. You can also include the time of day for precise calculations.
How do I convert a Julian Date to a calendar date?
Enter the JD value in the JD → Date mode. The calculator will convert it to a Gregorian (or Julian) calendar date with the time of day.
What is the day of the week for a given Julian Date?
The day of the week can be found using the formula: (JD + 1) mod 7, where 0 = Monday, 1 = Tuesday, ..., 6 = Sunday. The calculator does this for you automatically.
What is the Fliegel-Van Flandern algorithm?
The Fliegel-Van Flandern algorithm is a compact method for converting between Gregorian calendar dates and Julian Day Numbers. It was published in 1968 and is widely used in computer programs.
What is the difference between the Julian calendar and the Gregorian calendar?
The Julian calendar was introduced by Julius Caesar in 45 BCE. The Gregorian calendar was introduced by Pope Gregory XIII in 1582 to correct the drift in the Julian calendar. The Gregorian calendar is now the standard worldwide.
What is the current Julian Date?
The current Julian Date changes every day. You can use this calculator with today's date to find the current JD. The JD at noon UT is approximately 2,460,000 for the 2020s.
How do I convert JD to MJD?
MJD = JD − 2400000.5. For example, JD 2460409.0 corresponds to MJD 60408.5.
What is the Julian Date for January 1, 2000 at noon?
January 1, 2000 at 12:00 UT is JD 2451545.0. This is the J2000.0 epoch used in astronomy.
What is the Julian Date for April 8, 2024?
April 8, 2024 at 12:00 UT is approximately JD 2460409.0. This date is significant as it was a total solar eclipse.
What is the difference between JD and JDN?
JD (Julian Date) includes the fractional part of the day, while JDN (Julian Day Number) is the integer part only. JDN is the count of whole days since the epoch.
What is the earliest date that can be represented with JD?
The JD system can represent dates from about 4713 BCE to at least 9999 CE. The calculator supports years from -4713 to 9999.
Why is JD used in astronomy?
JD is used in astronomy because it avoids the complexities of calendars with months, leap years, and time zones. It provides a continuous, uniform time scale that is easy to calculate with.