Julian date calculator
Pick a date, get its Julian date.
What is a Julian date?
Two different things share the name. In everyday use it is the year plus the day of the year, like 26270 for September 27, 2026 (day 270) — USDA-graded egg cartons, for example, carry the day of the year the eggs were packed. In astronomy it is the Julian Date, a continuous count of days since noon on January 1, 4713 BC.
How do I read a YYDDD code?
The first two digits are the year and the last three the day of the year: 26001 is January 1, 2026, and 26365 is December 31, 2026.
What is the astronomical Julian Date?
A continuous day count used to date astronomical events; each day starts at noon UT. January 1, 2000 at 12:00 UT is JD 2451545.0. The U.S. Naval Observatory defines it as a running count of days and fractions that began at noon Universal Time on January 1, 4713 BC, in the Julian calendar.
What is the Modified Julian Date?
MJD = JD − 2,400,000.5, a shorter count that starts at midnight. January 1, 2000 at 12:00 UT is MJD 51544.5.
Which time should I enter?
Universal Time (UT) in 24-hour form. It matters only for the astronomical JD; the YYDDD code depends on the date alone.
Worked example: September 27, 2026
Day of the year: January to August hold 31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 = 243 days, and 243 + 27 = 270. The everyday codes are therefore 26270 (YYDDD) and 2026-270 (the ISO 8601 ordinal date, which writes the year in full and the day with three digits). For the astronomical value, Meeus’s formula for Gregorian dates uses A = ⌊2026 ÷ 100⌋ = 20 and B = 2 − A + ⌊A ÷ 4⌋ = −13, then JD = ⌊365.25 × (2026 + 4716)⌋ + ⌊30.6001 × (9 + 1)⌋ + 27 + B − 1524.5 = 2461310.5 at 0:00 UT. The same date at 12:00 UT is JD 2461311.0, and the Modified Julian Date at midnight is 61310.0.
Why does the Julian Date end in .5 at midnight?
Because the count begins at noon. A calendar day that starts at 0:00 UT falls in the middle of a Julian day, so its JD ends in .5, while 12:00 UT gives a whole number such as 2461311. The Modified Julian Date shifts the origin by 0.5 so that it changes at midnight like a civil date.
Common mistakes with Julian dates
First, mixing up the Julian calendar (Julius Caesar’s 45 BC calendar) with the Julian Date (a day count); the Naval Observatory points out that they are separate systems. Second, assuming day 366 exists every year: it appears only in leap years, such as 2028, where December 31 is 28366. Third, entering local time for the astronomical value. JD is tied to Universal Time, so a local evening can already be the next UT day; convert first. Fourth, dropping the century in a YYDDD code: 26270 could belong to 1926, 2026 or 2126, so keep the century when the data span more than one. Fifth, counting the day of the year by hand in a leap year: from March 1 on, every date is one number higher than in a common year.
Sources: U.S. Naval Observatory, Astronomical Applications Department, “Julian Date” (definition of JD and MJD); Meeus, J., Astronomical Algorithms, 2nd ed., chapter 7 (Julian day); ISO 8601 (ordinal dates, YYYY-DDD). Related: day of the year calculator, date calculator, Unix timestamp converter, leap year calculator and week number calculator.