Julian Date Converter

No. They share a name and almost nothing else. The Julian Day Number (JDN) is an astronomical day count since November 24, 4714 BC in the proleptic Gregorian calendar and runs into the millions. A Julian date on a food package, lot code, or US Department of Defense form is often an ordinal day paired with a 2-digit year. This converter handles JDN and ISO YYYY-DDD ordinal dates, but it does not parse the compact YYDDD code.

Scaliger published his Julian period in 1583 and chose the date so three astronomical and civil cycles all aligned on day one. The 28-year solar cycle, the 19-year Metonic lunar cycle, and the 15-year Roman indiction cycle multiply to 7980 years. He worked backward from 1582, the last point all three agreed, to find the previous alignment, which lands on what we now call November 24, 4714 BC in the proleptic Gregorian calendar. The starting point is intentionally earlier than any recorded historical date, so every observed event in human history has a positive JDN.

Modified Julian Date is related to the continuous Julian Date by MJD = JD - 2,400,000.5. The integer Julian Day Number (JDN) labels the day containing a given instant, so JDN and JD are not the same quantity. The Smithsonian Astrophysical Observatory introduced MJD in 1957 for satellite tracking. It uses fewer digits and starts each date at civil midnight rather than noon. MJD 0 corresponds to the start of November 17, 1858.

ISO 8601 writes the ordinal date as YYYY-DDD, where DDD is a zero-padded day of year from 001 to 365 or 366. So 2024-060 is February 29, 2024, and 2024-366 is December 31, 2024. The format is unambiguous, sorts correctly as a string, and avoids the month-day order confusion between US and European conventions. The DoD short form drops the century and writes the same date as 24060, which is fine inside a controlled system but ambiguous between 1924 and 2024 in the wild.

Leap years follow the Gregorian 4/100/400 rule: divisible by 4 is a leap year, except divisible by 100 is not, unless also divisible by 400. So 2024 and 2000 are leap years, but 1900 and 2100 are not. In a leap year, February 29 is day 60, March 1 is day 61, and the year has 366 days total. In a non-leap year, the converter rejects February 29, February 28 is day 59, and March 1 is day 60. The JDN algorithm applies proleptic Gregorian rules from 4714 BC through 9999 AD.