Convert between Gregorian calendar dates (YYYY-MM-DD) and Julian Day Numbers (JDN). Useful for astronomy, history, and chronological calculations.
Julian Day Number (JDN) Converter
Convert between standard Gregorian calendar dates and the Julian Day Number (JDN), a continuous count of days used primarily in astronomy.
Note: This calculates the astronomical JDN, not the 'Ordinal Date' (YYYYDDD format).
Convert Gregorian Date to JDN
Convert JDN to Gregorian Date
What is the Julian Day Number (JDN)?
The Julian Day Number (JDN) is a continuous count of days starting from noon Universal Time (UT/UTC) on January 1, 4713 BC, according to the proleptic Julian calendar. It was proposed by Joseph Scaliger in the 16th century but named after his father, Julius Scaliger (or possibly after the Julian calendar). It's widely used in astronomy and historical chronology because it provides an unambiguous way to identify specific days and calculate intervals between them, avoiding the complexities of different calendar systems (Julian, Gregorian), leap years, and month lengths.
Each JDN represents a whole day, starting at noon UTC. Fractions of a day can be used to represent specific times (often called Julian Date, JD), but this JDN converter focuses on the integer day number.
Key Uses:
- Calculating time intervals between astronomical events.
- Simplifying date calculations in software.
- Comparing dates across different historical calendar systems.
- Used as a standard in astronomical almanacs and catalogs.
Use this julian date converter to easily switch between the familiar Gregorian to Julian day number format and back (Julian to Gregorian). It's a helpful tool for students, astronomers, historians, and programmers needing precise date calculation.
JDN vs. Ordinal Date
Do not confuse JDN with the 'Ordinal Date' format (also sometimes called 'Julian Date' in some industries like programming or manufacturing), which represents a date as Year and Day-of-Year (YYYYDDD). For example, January 1st, 2025 is 2025001, and February 1st, 2025 is 2025032. This tool calculates the astronomical JDN.
Frequently Asked Questions (FAQs)
What does JDN 0 correspond to?
JDN 0 corresponds to the 24-hour period starting at noon UTC on January 1, 4713 BC in the proleptic Julian calendar.
Why does the Julian Day start at noon?
This was chosen primarily for astronomical convenience. It ensured that all observations made during a single night (from dusk till dawn) would fall under the same Julian Day Number.
Is this the same as the Julian calendar?
No. The Julian calendar is a historical calendar system introduced by Julius Caesar. The Julian Day Number is a continuous day count system *named* in reference to it but distinct in function.
How accurate are the calculations?
This converter uses standard, widely accepted algorithms for converting between Gregorian dates (after its adoption, typically 1582 or later, though algorithms work proleptically) and JDN. The calculations should be accurate for standard usage.
What is the difference between JD and JDN?
JDN (Julian Day Number) is typically the integer part, representing the whole day count since the epoch. JD (Julian Date) often includes a fractional part representing the time of day (e.g., 2451545.0 corresponds to noon UTC on Jan 1, 2000, while 2451545.5 corresponds to midnight UTC at the start of Jan 2, 2000). This tool focuses on calculating the JDN integer.
