Ordinal Dates and Day-of-Year Format

Ordinal Dates (Day of Year)

An ordinal date expresses a date as the sequential day number within a year, ranging from 1 (January 1) to 365 or 366 (December 31 in a leap year). This is a compact and useful representation in several specialized fields.

ISO 8601 Ordinal Date

2026-059     # February 28, 2026 (the 59th day)
2026-001     # January 1
2026-365     # December 31

Format: YYYY-DDD where DDD is the zero-padded day of year.

Day-of-Year Table

Date	Day of Year	Notes
January 1	001
February 1	032
February 28	059
February 29	060	Leap years only
March 1	060 or 061	Depends on leap year
July 1	182	Midpoint of the year
December 31	365 or 366

Calculating Day of Year

// JavaScript
const d = new Date(2026, 1, 28);
const start = new Date(d.getFullYear(), 0, 0);
const dayOfYear = Math.floor((d - start) / 86400000); // 59

# Python
from datetime import date
d = date(2026, 2, 28)
day_of_year = d.timetuple().tm_yday  # 59
# or with strftime
d.strftime("%j")  # "059"

// Go
t := time.Date(2026, 2, 28, 0, 0, 0, 0, time.UTC)
dayOfYear := t.YearDay() // 59

Format Tokens

Language	Token	Output
Python	%j	059 (zero-padded to 3 digits)
PHP	z	58 (0-indexed, no padding)
Java	DDD	059
Ruby	%j	059

Note: PHP's z returns 0-indexed (January 1 = 0), unlike all other languages which use 1-indexed.

Applications

Aviation (ICAO/FAA) Flight plans use ordinal dates in the format YYDDD:

26059 = February 28, 2026

Military (DTG — Date-Time Group) Military date-time groups sometimes use Julian dates for brevity.

Scientific Data Satellite imagery, weather data, and astronomical observations often use ordinal dates for file naming:

LANDSAT_2026059_B4.tif  # Landsat Band 4, day 59 of 2026

Manufacturing Production lot codes often encode the manufacture date as YYDDD:

Lot 26059 = Manufactured on the 59th day of 2026