Day of Year Calculator - Ordinal Date, ISO 8601 Format

Every date has a number between 1 and 366

The ordinal date - also called the day-of-year number - counts calendar days from January 1 (day 1) to December 31 (day 365, or 366 in leap years). This calculator converts any date to its ordinal number and back. It also reports how many days have passed, how many remain, the year progress as a percentage, and the ISO 8601 ordinal format used in aviation, military logistics, and data systems.

How to find the day of the year - step by step

Calculator mode - choose "Date to Day of year" to find the ordinal number, "Day of year to Date" for the reverse, or "Today's date" for a quick check.
Date (first mode) - pick a date from the calendar or type it as YYYY-MM-DD.
Year + Day number (second mode) - enter the year and an ordinal number between 1 and 365 (or 366).
Read the results - day number, full date, days passed, days remaining, year progress, and ISO 8601 format.

Key ordinal dates to remember

Date	Day (common)	Day (leap)	Note
January 1	1	1	Always day 1
March 1	60	61	+1 shift after Feb 29
April 10	100	101	The 100th day
July 2	183	184	Midpoint of the year
October 1	274	275	Start of Q4
December 31	365	366	Last day of year

Where ordinal dates are used

Field	How it is used
Aviation	Flight plans, weather reports (METARs) and NOTAMs use Julian dates (YYDDD format)
Military / NATO	Date-time groups in logistics and communications
Manufacturing	Lot codes and expiry dates on food packaging (e.g. "26173" = 2026, day 173 = June 22)
Data / ISO 8601	The YYYY-DDD format is unambiguous across locales - no month/day order confusion

Practical examples

Example 1 - date to day number
March 15, 2026. January has 31 days, February has 28 (2026 is not a leap year), plus 15 days of March = day 74. ISO format: 2026-074.

Example 2 - day number to date
Day 200 of 2026. Counting forward: Jan 31 + Feb 28 + Mar 31 + Apr 30 + May 31 + Jun 30 = 181. Remaining: 200 - 181 = 19. Result: July 19, 2026.

Example 3 - leap year shift
March 1, 2024 (leap year). January 31 + February 29 + 1 = day 61. In a common year, March 1 would be day 60.

Example 4 - manufacturing lot code
A food package shows lot code 26173. That means year 2026, day 173 = June 22, 2026.

Example 5 - year progress
On September 1, 2026 (day 244), you have completed 66.85% of the year. 121 days remain until December 31.

FAQ

What day of the year is April 10?

In a common year (365 days), April 10 is day 100. In a leap year, it is day 101 because February has 29 days instead of 28.

Is day-of-year the same as Julian date?

Not exactly. The term "Julian date" in astronomy refers to a continuous count of days since January 1, 4713 BC. In everyday use (aviation, manufacturing), "Julian date" often means the ordinal day of the year (1-366), which is technically called the ordinal date. This calculator uses the ordinal date definition.

What is the ISO 8601 ordinal date format?

ISO 8601 defines the ordinal date format as YYYY-DDD, where YYYY is the four-digit year and DDD is the three-digit day number (zero-padded). Example: March 15, 2026 = 2026-074.

How do I decode a lot code like 26173?

The first two digits are the year (26 = 2026) and the last three are the ordinal day (173). Use the "Day of year to Date" mode: enter year 2026 and day 173. Result: June 22, 2026.

Does the leap year affect day numbers after February?

Yes. Every date from March 1 onward has a day number that is 1 higher in a leap year than in a common year. January and February dates are the same in both types of years.

What is the midpoint of the year?

In a common year (365 days), the midpoint is noon on July 2 (day 183). In a leap year, the midpoint is midnight between July 1 and July 2 (day 183/184).

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