The ordinal date — day 1 to 365, or 366 in a leap year. Used in aviation, logistics, manufacturing lot codes and lab notebooks, and computed here in both directions.
Where the leap day actually shows up
An ordinal date is a plain count: 1 January is day 1, 31 December is day 365, or 366 in a leap year. The only place the leap day changes anything is from 1 March onward. In a common year 1 March is day 60; in a leap year it is day 61, and every date after it shifts by one. Dates in January and February are unaffected, which is why an off-by-one in leap handling can look correct for two months and then break.
Both directions
The reverse lookup matters more than it sounds. Manufacturing lot codes, aviation records and a good deal of scientific data are stamped with a year and a day number rather than a date, and reading them back requires knowing whether that year was a leap year. Day 60 of 2024 is 29 February; day 60 of 2026 is 1 March. Enter a number and a year above and the date comes back; enter a date and the number does.
This is not the Julian date
The ordinal day number is often called the Julian day, and in aviation and manufacturing that is exactly what people mean by it. Astronomers mean something quite different: a continuous count of days since 4713 BC, currently in the two-and-a-half-millions, used so that any two instants can be subtracted without touching a calendar. If a form asks for a Julian date and expects three digits, it wants the ordinal day number on this page. If it expects seven, it wants the astronomical one.
Questions
What day of the year is it today?
The box above opens on today's date and the answer is the first figure in the strip. The count runs from 1 on 1 January to 365 on 31 December, or 366 in a leap year. The days-remaining figure beside it counts forward to the end of the year and does not include the current day, so on 31 December it reads zero.
Why is 1 March day 60 in some years and 61 in others?
Because 29 February sits in front of it. In a common year January and February use up 59 days, so 1 March is day 60. In a leap year they use 60, so 1 March is day 61 and every date after it shifts by one. This is the only place in the whole year where the leap day changes an ordinal number, which is why a calculator with a leap-year bug can look perfectly correct through January and February.
Is this the same as a Julian date?
It depends who is asking. In aviation, manufacturing and food packaging, \u201cJulian date\u201d normally means exactly this: the day number within the year, usually as three digits, often with a year digit in front. Astronomers use the term for something entirely different — a continuous count of days since 4713 BC, currently over 2,460,000, designed so that any two instants can be subtracted without reference to a calendar. A three-digit field wants the number on this page.
How do I read a lot code like 6236?
That format is usually one year digit followed by three day digits, so 6236 is day 236 of a year ending in 6. Enter 236 and the year in the boxes above to get the date. Be careful with the year digit: it only narrows the year to one in ten, so 6236 could be 2016 or 2026, and the date it maps to differs because leap years fall differently. If the product has a shelf life shorter than a decade the ambiguity usually resolves itself.
Does the day number restart at the same date every year?
Only for dates before 1 March. From 1 March onward a given date is one day number in a common year and one higher in a leap year, so a date and a day number are not interchangeable without knowing the year. This is the reason the year box above is not optional for the reverse lookup.
Dates and Days