Julian Day Calculator
This calculator converts a standard calendar date into its Ordinal Date (day of the year) and its corresponding Julian Day Number (JDN). The Ordinal Date is a simple count of days from 1 to 365 (or 366 in a leap year). The Julian Day Number is a continuous count of days since the beginning of the Julian period, primarily used in astronomy.
Result for December 1, 2025
335Ordinal Date (Day of Year)
2,461,011Julian Day Number
The Julian Day Number (JDN) is the integer assigned to a whole solar day in the Julian day count starting from noon Universal Time, with day 0 being assigned to the day starting at noon on Monday, January 1, 4713 BC.
How It's Calculated
Ordinal Date vs. Julian Day Number
These two values, while similar, serve different purposes:
- Ordinal Date: This is a simple count of the day within the current year. January 1st is 1, February 1st is 32, and so on, up to 365 or 366 for a leap year. It's often represented as YYYY-DDD.
- Julian Day Number (JDN): This is a continuous count of days that has been running for thousands of years, starting from January 1, 4713 BC. It's a single, unambiguous number for any given date, which makes it extremely useful for astronomers and historians to calculate the time between two distant events.
Example Calculation
For the date you selected, December 1, 2025:
- Ordinal Date: The calculator simply counted the number of days from the beginning of the year 2025 to your selected date, which is 335.
- Julian Day Number: A complex astronomical formula was used, involving the year, month, and day, to calculate the total number of days that have passed since the starting point in 4713 BC. This results in the Julian Day Number of 2,461,011.