Day of Year Calculator

Day of year (sometimes called ordinal date or Julian day) is the count of days since January 1, ranging from 1 to 365 (or 366 in leap years). January 1 is day 1; December 31 is day 365 or 366. Used in scheduling, agriculture, accounting periods, and certain scientific data formats.

Enter a calendar date to see its day-of-year value, days remaining in the year, and the percentage of the year completed. Leap year handling adjusts February 29 onward by one day relative to non-leap years.

The arithmetic is a lookup of cumulative month lengths. January adds 31 days, February 28 (or 29), March 31, April 30, and so on. Day-of-year for any date = sum of days in completed months + day of month. For March 15 in a non-leap year: 31 + 28 + 15 = day 74. In a leap year: 31 + 29 + 15 = day 75. The leap year rule: divisible by 4 except century years not divisible by 400. So 2000 was leap, 1900 wasn't, 2400 will be. The calculator applies the rule and shifts dates from February 29 onward by one day in leap years versus regular years.

A worked example. Date: October 15, 2026. 2026 is not a leap year (2024 was the most recent leap, 2028 is the next). Cumulative through September: 31+28+31+30+31+30+31+31+30 = 273. Add 15: day 288. Days remaining in year: 365 - 288 = 77. Percentage of year completed: 78.9 percent. The same date in 2024 (a leap year): 31+29+31+30+31+30+31+31+30 = 274. Add 15: day 289. Same date, different ordinal — by one day.

Limitations and naming pitfalls. The most common confusion is with Astronomical Julian Date, which is something else entirely. Astronomical JD is a continuous count of days since 4713 BCE noon UTC; as of 2026 it sits around 2.46 million. Some scientific software conflates the two, producing off-by-millennia bugs. ISO 8601 supports an ordinal date format (YYYY-DDD, e.g., 2026-127 for May 7), but it's rarely used in human-facing contexts. ISO week dates (YYYY-Www-D) are a different ordinal scheme aligned to weeks rather than days from January 1, and have their own edge cases — week 1 is the week containing January 4, which in some years means dates in late December belong to week 1 of the next year. Day-of-year is convenient for budgeting (which percent of the year is done), weather modeling (climatological day-of-year), and any application where a date is a position on a continuous timeline rather than a calendar slot.