Two things people mean by "Julian date"
The phrase has two common meanings, and mixing them up causes errors. The first is the Julian day number, JDN, a plain count of days used in astronomy and software since a fixed starting point long ago. 1 January 2000 is JDN 2451545. The Modified Julian Date, MJD, is a shorter version of the same count, and that day is MJD 51544.
The second meaning is the one used on food packs, factory labels and military paperwork: the year plus the day of the year. 1 February 2027 is the 32nd day, so it is written 2027-032 or in the five-digit YYDDD form as 27032. This has nothing to do with the astronomical count above.
Enter a calendar date and the page shows all of these side by side. Type a JDN into the reverse box to get the calendar date it names. The top of the page also lists today's values from your device date, which is handy when you are checking a label.
Neither of these is the old Julian calendar that was replaced by the Gregorian one. This tool does not convert dates to that calendar. It uses today's Gregorian dates throughout and does not apply any historical calendar switch.
Worked examples
Questions about this tool
Is a Julian date the same as the Julian calendar?
No. The Julian calendar is an old calendar system. A Julian date is either a running day count or a year plus day-of-year code.
Why is the five-digit code 24366 for 31 December 2024?
24 is the year and 366 is the day, because that year had 366 days.
What is the difference between JDN and MJD?
MJD is the JDN minus 2400000.5, which gives a smaller number that starts at midnight rather than noon.