Convert between Gregorian dates, Julian Day Number (JDN), Modified Julian Date (MJD), and ordinal date (ISO 8601 YYYY-DDD or DoD YYDDD). Uses the Fliegel and Van Flandern (1968) algorithm.
Julian date converter
JDN is the astronomical day count since November 24, 4714 BC (proleptic Gregorian). Ordinal date is the day-of-year ISO 8601 format, also called "Julian date" in DoD and agriculture.
Any Gregorian calendar date from 4713 BC through 9999 AD.
Gregorian 2026-06-19 to JDN
2026-06-19
Friday. Ordinal date 2026-170. Non-leap year.
Julian Day Number
2,461,211
Modified Julian Date
61,210.5
Day of year
170
ISO ordinal
2026-170
Weekday
Friday
Leap year?
No
JDN uses the algorithm from Fliegel and Van Flandern (Communications of the ACM, October 1968). MJD = JDN - 2400000.5. Ordinal date follows ISO 8601 (YYYY-DDD).
Frequently Asked Questions about the Julian Date Converter
Is the astronomical Julian Day Number the same thing as a Julian date in agriculture or the DoD?
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, with values like 2451545 for the J2000.0 epoch. A Julian date on a food package, lot code, or US Department of Defense form is an ordinal date: the day of year (1 to 366), often paired with a 2-digit year, so 24365 means day 365 of 2024 (December 30). This converter handles both formats and labels which one you get.
Why did Joseph Scaliger pick November 24, 4714 BC as JDN 0?
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.
What is the Modified Julian Date and why do astronomers use it?
MJD equals JDN minus 2400000.5. The Smithsonian Astrophysical Observatory introduced it in 1957 for satellite tracking, and NASA, the IAU, and most modern astronomy software use it instead of raw JDN. Two reasons. First, MJD fits in fewer digits, so 60000 is easier to log than 2460000.5. Second, MJD starts the day at civil midnight rather than at noon, so an MJD timestamp lines up with the calendar day people work in. MJD 0 corresponds to the start of November 17, 1858.
How does the ISO 8601 ordinal date format work?
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.
How does the converter handle leap years and the Feb 29 boundary?
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, February 29 does not exist (the converter rejects it), February 28 is day 59, and March 1 is day 60. The JDN algorithm uses the same Gregorian rules across its entire supported range, from 4713 BC through 9999 AD.