dayofyear

The day of year, often abbreviated as dayofyear, is the ordinal position of a date within its calendar year. It ranges from 1 for January 1 to 365, or 366 in leap years, with February 29 contributing the extra day.

In computing and data analysis, dayofyear is commonly exposed as a function or property that returns this ordinal value from a date. Names and syntax vary by language: some SQL dialects provide a DAYOFYEAR function, Python libraries expose dayofyear or dt.dayofyear, and Oracle can derive it with date formatting or date_part('doy', date). In spreadsheets, day-of-year can be computed by subtracting January 1 of the same year from the date and adding 1.

Leap years affect the dayofyear scale by introducing a 366th day. The typical rule is that years divisible by 4 are leap years, but years divisible by 100 are not unless they are also divisible by 400. This determines whether December 31 is day 365 or 366 and influences the numbering of days after February.

Common uses of dayofyear include aligning events across years, seasonality analysis, and time-series indexing that requires a consistent year-relative counter. It is also used in fiscal or operational planning to compare periods by their position within a year, rather than by absolute calendar dates.

Related concepts include ordinal date and Julian day, which refer to similar notions of counting days within a year or since a fixed epoch. Dayofyear is a practical, language- and context-dependent representation of calendar progress through a year.