Today’s Julian date

Three different things called “Julian date”

Almost every confusion about Julian dates comes from one name being used for three unrelated systems. On Monday, November 2, 2026, for example, all of these were correct answers to “what is the Julian date?”:

NOAA’s glossary says it outright: some scientists use “julian day” for the day number within the current year, which differs from the astronomical Julian Day. So when a form, label or colleague asks for a Julian date, the format tells you which one: five or seven digits like 26306 or 2026306 are day-of-year codes; a seven-digit number starting with 24 and often ending in .5 is an astronomical Julian Day.

Day-of-year codes: YYDDD, YYYYDDD, YDDD and CYYDDD

The “Julian date” of industry is simply the year plus the day of the year, counted from 001 on January 1 to 365 on December 31 (366 in leap years). The variants differ only in how much of the year they keep:

• YYDDD (26306): two-digit year and three-digit day. The most common form, often just called the Julian date.
• YYYYDDD (2026306): four-digit year, so there is no century ambiguity. Written with a hyphen (2026-306), it is one of the ISO 8601 date formats (RFC 3339, Appendix A).
• YDDD (6306): only the last digit of the year. The US Department of Defense builds disposal turn-in document numbers (DTIDs) from a 6-digit DoDAAC, the last digit of the year, a three-digit Julian date and a four-digit serial number (32 CFR 273.7). A single digit repeats every ten years, so the context must supply the decade.
• CYYDDD (126306): a century digit (0 = 1900s, 1 = 2000s) in front of YYDDD. Oracle’s JD Edwards guides use this format: 0 for the 20th century, 1 for the 21st, so June 1, 1999 is 099152 and June 1, 2000 is 100153 (2000 was a leap year, so the day number is one higher).
• DDD alone: on egg cartons and many lot codes the year is left out. USDA-graded shell eggs must be lot-numbered with the consecutive day of the year on which they were packed (7 CFR 56.37, whose example is 132); other numbering systems can be approved in writing.

Why use a day number at all? It sorts correctly, takes few characters, and makes date arithmetic easy: shelf life of 90 days from day 306 is day 396, which is day 31 of the next year. That is the appeal for business systems and lot codes.

Working out the day number by hand

Add the day of the month to the day number of the month’s first day, minus one. November 1 is day 305 in a common year, so November 2 is day 306. From March on, leap years are one ahead:

The astronomical Julian Day (JD)

The US Naval Observatory defines the Julian Date as a continuous count of days and fractions of a day from Greenwich mean noon (12h UT1) on January 1, 4713 BC of the Julian calendar, which astronomers number −4712. It has no months, weeks or leap years, which makes it ideal for computing the time between two observations: subtract one JD from the other.

• Days start at noon. 0h Universal Time is always a .5 fraction: November 2, 2026 at 0h UT is JD 2461346.5, and noon that day is JD 2461347.0. USNO’s own examples: January 1, 1978 at 0h is 2443509.5; July 21, 1978 at 15h is 2443711.125.
• The Julian Day Number (JDN) is the integer part of the JD. The number usually given for a calendar date (2461347 for November 2, 2026) is the one in force from noon of that date.
• Modified Julian Date (MJD) = JD − 2400000.5. It starts at midnight and is five digits for today’s dates (61346 for November 2, 2026); MJD 0 is November 17, 1858.
• J2000.0, the reference epoch of modern astronomy, is JD 2451545.0 in Terrestrial Time: January 1, 2000, 12h TT, which was 11:58:55.816 UTC. For precise work, state the time scale (TT or UT) along with the JD.
• Weekday from the JD: the remainder of the JDN divided by 7 gives the day of the week (0 = Monday): 2461347 mod 7 = 0, and November 2, 2026 was a Monday.
• Unix time converts with a fixed offset: JD = Unix seconds ÷ 86,400 + 2440587.5 (see Unix time).

The algorithm behind the converter is Fliegel and Van Flandern’s (1968), as published by USNO, which turns a Gregorian date into a JDN with integer arithmetic. It applies to Gregorian dates; for dates before a country adopted the Gregorian calendar, convert from the Julian calendar instead. The calendar change itself leaves no gap in the count: Thursday, October 4, 1582 (Julian) began at JD 2299159.5 and the next day, Friday, October 15, 1582 (Gregorian), at JD 2299160.5.

The Julian calendar

The third meaning is the calendar Julius Caesar introduced in 46 BC, in effect from 45 BC: 365 days with a leap day every fourth year, so an average year of 365.25 days. That is about 11 minutes longer than the tropical year, which is why Pope Gregory XIII’s reform of 1582 dropped ten dates (Thursday, October 4 was followed by Friday, October 15) and stopped counting century years as leap years unless divisible by 400. The Gregorian average is 365.2425 days.

The two calendars drift apart by three days every 400 years. From the Julian leap day of 1900 (Gregorian March 13, 1900) until Gregorian March 13, 2100, the Julian calendar is 13 days behind: November 2, 2026 in our calendar is October 20, 2026 in the Julian one, and Julian December 25, 2026 falls on Gregorian January 7, 2027. Before 1900 the gap was 12 days (back to 1800), 11 days (1700–1800) and 10 days (1582–1700); it becomes 14 days in 2100.

Countries switched at very different times, so historical dates need care. Great Britain and its colonies changed in 1752 under the Calendar (New Style) Act 1750: Wednesday, September 2, 1752 was followed by Thursday, September 14, eleven days being dropped, and the legal year, which had begun on March 25, began on January 1 from 1752. Later adoptions in the US Social Security Administration’s table include Bulgaria (April 14, 1916), Russia, Estonia and Latvia (February 14, 1918), Romania (April 15, 1919), Greece for civil records (March 1, 1923) and Turkey (January 1, 1926).

Common mistakes

• Sending a JD when a day-of-year code was wanted, or the reverse. If a field is five characters wide, it wants YYDDD.
• Forgetting leap years. Day 060 is March 1 in a common year and February 29 in a leap year. Any code from 060 on shifts by one.
• Two-digit years. 26306 is 2026 to almost everyone today, but a system can read it as 1926. YYYYDDD and CYYDDD avoid that.
• Mixing local and UTC days. Day-of-year codes follow the local date; JD and MJD follow Universal Time. Near midnight the two can disagree by a day.
• Day 000. It does not exist: January 1 is day 001.