# Sorting out Julian Day, Julian Date, Julian Day number, Julian Day Calendar, and Julian Day Table

In this answer I mention day number which is 1 on the first day of each calendar year (January 1) and increments to 365 or 366 on December 31 of that year.

There was an edit proposed, which included a sentence explaining that this could be called the Julian Day. I left a message (elsewhere) to the author of the edit I rejected saying:

Hi, I rejected your edit on this answer because Julian day is not correct. JD is monotonic and is currently 2,458,643.06

to which the author replied

@uhoh many thanks for the comment! However, there is such a thing as Julian Day of the year. See this NASA web page: https://landweb.modaps.eosdis.nasa.gov/browse/calendar.html

Looking further I saw several similar pages: 1, 2, 3, 4, 5 Some are called Julian Day Calendar and the rest are called Julian Day Table, but each is a table of what I have defined as day numbers, 1 on January 1, and 365 or 366 on December 31.

Could someone help me sort out all of the possible, acceptable terms for the following numbers corresponding to yesterday which include some form of Julian:

Please be generous; include usages that are less common or have fallen out of use but still have some historical use.

• 2,458,643 (integer)
• 2,458,643.06 (float)
• 160 (day number for June 8 on non-leap years)
• June 8, 2019
• Doesn't answer your question + probably doesn't help, but the Unix cal command has this option: -j, --julian Display Julian dates (days one-based, numbered from January 1). As a note, there might be a difference between Julian day and Julian date – barrycarter Jun 9 '19 at 16:13
• @barrycarter cool! I see cal -j builds a little calendar with day numbers for June (using bash in MacOS), and -jy does it for the whole year. (other options too). Yes this is exactly the kind of thing I'm looking for. – uhoh Jun 9 '19 at 16:18
• Ordinal Date seems to be a possible term for the year and day-of-year format. (YYYY-DDD) ISO 8601 uses the term Ordinal Date in their standardized date formats. – FSimardGIS Jun 9 '19 at 17:56

The Julian Date (JD) is the real number of days since 12:00 UT, January 1, 4713 BCE (proleptic Julian calendar). The Julian Day Number (JDN) is floor(JD), the nearest integer ≤ JD.

The Modified Julian Date (MJD) is JD - 2400000.5, or the real number of days since 1858-11-17 0:00 UT. The IAU officially approves either JD or MJD as long it's clear which quantity is given.

A number of 366 or less in a bureaucratic or informatics context is a day number or day of the year. In his MJD explainer, the late USNO Time Service director Gernot Winkler strongly opposed calling this a "Julian" date:

The MJD (and even more so the JD) has to be well distinguished from this day of the year (DOY). This is also often but erroneously called Julian Date, when in fact it is a Gregorian Date expressed as number of days in the year. This is a grossly misleading practice that was introduced by some who were simply ignorant and too careless to learn the proper terminology. It creates a confusion which should not be taken lightly. Moreover, a continuation of the use of expressions "Julian" or "J" day in the sense of a Gregorian Date will make matters even worse. It will inevitably lead to dangerous mistakes, increased confusion, and it will eventually destroy whatever standard practices exist.

Expressing the example date of 2019-06-08 13:26 UT in various forms:

• Julian day number: 2458643
• Julian date: 2458643.06
• Modified Julian date: 58642.56
• Day of year: 159
• Ordinal date: 2019-159
• +1 for a thorough and well-sourced answer, thank you! – uhoh Jun 10 '19 at 7:05
• OpenVMS use the MJD as it's epoch. – RonJohn Jun 12 '19 at 21:03