How to Calculate Day of Year

A practical method

The day of year is an ordinal number that counts January 1 as day 1. Each following date adds the lengths of the preceding months and the current day number. Dates after February need one extra day in a leap year, which is the main edge case in manual calculations.

1. Check whether the year is a leap year: Years divisible by 4 are leap years, except century years unless they are also divisible by 400.
2. Add completed months: Sum the number of days in every month before the target month.
3. Add the current day: Because January 1 is day 1, add the calendar day number without subtracting one.
4. Validate the range: The result must be 1–365 in a common year or 1–366 in a leap year.

Worked example

For March 1, 2028, add January's 31 days, February's 29 days, and 1 for March 1. The result is day 61. In 2027, February has 28 days, so March 1 is day 60.

Checks, edge cases, and common mistakes

• Do not confuse day of year with day of month or ISO week number.
• Remember that leap-year adjustment applies only to dates after February 28.
• Some systems use zero-based ordinal days; verify whether January 1 is represented as 0 or 1.
• Include the year whenever sharing an ordinal date because the leap-year status affects the result.