YYYYDDD

YYYYDDD is a calendar date format that encodes the year followed by the day of the year. In this scheme, the year is represented by four digits, and the day of year is a three-digit number from 001 to 365 (or 366 in leap years). The format is also known as an ordinal date and is used in meteorology, aviation, logistics, and data logging to provide a compact, easily sortable representation of dates.

The main advantage of YYYYDDD is that it sorts naturally by date when treated as a numeric value and remains unambiguous within the context of a specific year. It does not include time zone or time of day information, so it is best suited for daily records or batch identifiers. It is commonly found in weather reports, production scheduling, and file naming schemes where a concise, time-ordered code is beneficial.

Leap year considerations: DOY values range up to 366 in leap years. When interpreting or generating YYYYDDD values, it is necessary to know whether the year is a leap year to determine whether DOY 366 is valid (only in leap years).

Conversion between YYYYDDD and a Gregorian calendar date can be done by taking the year, determining January 1 of that year, and adding (DOY − 1) days. For example, 2025012 corresponds to January 12, 2025. 2024366 corresponds to December 31, 2024. To format a date back to YYYYDDD, compute the day of the year and pad the DOY to three digits, yielding a four-digit year plus a three-digit DOY.

Note that YYYYDDD is distinct from the astronomical Julian Day Number, which counts days from a fixed epoch and is not tied to a year-day format.