yearday

Yearday is a term used in calendrical and data-processing contexts to denote the ordinal position of a day within a calendar year. It ranges from 1 to 365 in common years and from 1 to 366 in leap years. The concept is equivalent to an ordinal date, and in ISO 8601 it is often written as YYYY-DDD, where DDD is the day of the year, ranging from 001 to 366.

To determine a yearday, one must know whether the year is a leap year and then sum the days in the months preceding the given date, adding the day of the month. In the Gregorian calendar, leap years occur every year divisible by 4, except that years divisible by 100 are not leap years unless they are also divisible by 400.

Common examples include January 1 being yearday 1, and December 31 being yearday 365 in non-leap years or 366 in leap years (for example, 2020-366 corresponds to December 31, 2020).

Uses of yearday include data logging, meteorology, astronomy, and software development, where a simple, month-independent count of days facilitates storage, sorting, interval calculations, and cross-year comparisons. Storing or exchanging dates as year plus yearday can be advantageous in systems with fixed-width fields or where human-friendly month names are unnecessary.

Limitations include its readability for human users and the need to correctly handle leap years. When presenting information to people, yearday values are often converted back to conventional month-day formats to aid comprehension.