Day of Year Calculator

The day of the year (also called the ordinal date) is a number from 1 to 365 (or 366 in a leap year) that indicates which day of the year a given date falls on. January 1 is day 1, January 2 is day 2, and so on through December 31, which is day 365 (or 366 in a leap year). This numbering system is useful in astronomy, the military, agriculture, and computing where a single number is more convenient than a month-day combination.

Click the "Today" button in the calculator above to instantly find today's day number. For reference, May 15 is day 135 in a non-leap year and day 136 in a leap year. The calculator automatically accounts for whether the selected year is a leap year. January 1 is always day 1, and December 31 is always the last day (365 or 366).

Add up the number of days in all complete months before the target month, then add the day of the month. For non-leap years: January = 31 days, February = 28, March = 31, April = 30, May = 31, June = 30, July = 31, August = 31, September = 30, October = 31, November = 30. Example: March 15 = 31 (Jan) + 28 (Feb) + 15 = 74. In a leap year, add 1 to any date from March 1 onward. For programming: day = Math.floor((date - Jan1) / 86400000) + 1.

Day 100 falls on April 10 in a non-leap year and April 9 in a leap year. After day 100, 265 days remain (264 in a leap year). Day 100 is sometimes noted as a milestone called "The 100th Day" in schools. Other common milestone days: day 1 = January 1, day 91 = April 1 (approximately end of Q1), day 182 = July 1 (mid-year), day 274 = October 1 (start of Q4), day 365 = December 31.

A leap year has 366 days instead of 365, with February 29 added. Leap years occur when a year is divisible by 4, except for century years not divisible by 400 (so 2000 was a leap year, 1900 was not, and 2100 will not be). For dates from January 1 through February 28, the day number is the same in both leap and non-leap years. From March 1 onward, every date's day number is 1 higher in a leap year. December 31 is day 365 in a non-leap year and day 366 in a leap year.

In computing and the US military, the ordinal date format YYYY-DDD is used, where DDD is the day of the year. For example, May 15, 2026 would be written as 2026-135. This format is defined by ISO 8601 and is sometimes called the "Julian date" in computing contexts (distinct from the astronomical Julian Day Number). It is used in COBOL mainframe systems, US military logistics, food manufacturing expiration codes, and some EDI (Electronic Data Interchange) standards. Converting from YYYY-DDD to a calendar date requires the same calculation as our Day Number to Date mode.

Agriculture uses Growing Degree Days (GDD) and day-of-year numbers extensively. Planting schedules, frost dates, and harvest windows are often expressed as day ranges. For example, "plant corn after day 120" means after April 30 in a non-leap year. Weather stations record temperature data by day number for comparison across years. The USDA uses ordinal dates for crop reports. Last frost dates are expressed as the average day number of the last frost in a region, making it easy to count days to planting time regardless of which day of the week the date falls on.

Percentage = (day number / total days in year) × 100. In a non-leap year, July 2 (day 183) is the exact midpoint: 183/365 ≈ 50.1%. In a leap year, July 2 (day 184) is also near the midpoint: 184/366 ≈ 50.3%. The exact midpoint of a non-leap year falls between July 2 and July 3. This calculator shows the year progress as a percentage bar in the results panel, updated for the exact date you enter.

Some commonly referenced day numbers in a non-leap year: Day 1 = Jan 1 (New Year). Day 32 = Feb 1. Day 60 = Mar 1. Day 91 = Apr 1 (end of Q1). Day 100 = Apr 10. Day 121 = May 1. Day 151 = Jun 1. Day 182 = Jul 1 (mid-year). Day 212 = Aug 1. Day 243 = Sep 1. Day 274 = Oct 1 (start of Q4). Day 305 = Nov 1. Day 335 = Dec 1. Day 365 = Dec 31. In a leap year, add 1 to any date from March 1 onward.

Food manufacturers often stamp a 3-digit Julian date (day of year) on packaging for lot tracking. A code of 135 means day 135 of the year. Switch to "Day Number to Date" mode, enter 135 and the year (usually the current year or the year printed alongside), and get the calendar date. Some codes use a 5-digit format: the first two digits are the year and the last three are the day. For example, 26135 = day 135 of 2026 = May 15, 2026. Canned goods, dairy, and fresh produce all commonly use this system.