Day of the Year Calculator
Find the ordinal day number (1-366) for any date, or convert a known day number back to a calendar date.
How this is calculated
The day number counts forward from January 1st (day 1) through the target date, including that date. To find it by hand, add up the actual day counts of every complete month before the target month, then add the day of the month. A leap year adds February 29th, so every date from March 1st onward lands one day number higher than the same calendar date in a non-leap year. For example, July 18, 2024 (a leap year) is day 200, while July 18, 2026 (not a leap year) is day 199.
Frequently asked questions
What is a day of the year number?
It's also called the ordinal date. It's a count of days from January 1st (day 1) through a given date, including that date. December 31st is day 365 in a common year, or day 366 in a leap year.
How do leap years affect the day number?
Every date from March 1st onward is one day number higher in a leap year than the same calendar date in a non-leap year, since February 29th adds an extra day before it. Dates in January and February aren't affected.
What is the ISO ordinal date format?
ISO 8601 expresses an ordinal date as YYYY-DDD, where DDD is the three-digit day number with leading zeros. For example, April 10, 2026 is written 2026-100.
Is the day of the year the same as a Julian date?
Informally, yes, many logistics and business contexts use "Julian date" to mean the ordinal day-of-year number. The true astronomical Julian Date is a different, continuous day count that has been running since 4713 BC and is currently over 2.4 million, not the 1-366 range this calculator shows. For the full breakdown of all three meanings, see What Is a Julian Date?
Sources
- FoundTheTool, Day of Year
- US Naval Observatory Astronomical Applications Department, Julian Date documentation
Built and maintained by Ready Utilities, founded by Cedrick Reese, a retired veteran and web developer. Every formula and convention on this site is checked against real, current sources before publishing. This calculator runs entirely in your browser, nothing you enter is sent anywhere. Spot something that looks wrong? Let us know.