Day of the Year Calculator
Find which numbered day of the year any date is (today by default), how many days remain and how much of the year has passed — or turn a day number or a Julian date code like 26166 back into a date — with ISO week, ordinal date and Julian Day Number.
How it works
day of year = days in the earlier months + day of the month (1 January = day 1) · days remaining = year length − day of year · elapsed % = day of year ÷ year length × 100 · Julian code YYDDD = last two digits of the year + day of year padded to 3 digits
Day-of-year, also called the ordinal date, counts 1 January as day 1, so it runs 1 to 365, or 1 to 366 in a leap year. It is widely used in science, aviation, manufacturing and IT logs: a “Julian date” on a lot code or a batch record is usually this number, often written as YYDDD (26166 for June 15, 2026) or YYYYDDD. The calculator works both ways — pick a date to get its number, enter a day number and a year to get the date, or paste a Julian code to decode it. Alongside the count it shows the days left and the share of the year gone, the ISO week number and ISO week date (week 1 is the week with the year’s first Thursday), the quarter, the Monday-to-Friday weekdays left, the astronomical Julian Day Number and Modified Julian Date, and a table with the first and last day number of every month.
Worked example
15 Jun 2026 follows 31 + 28 + 31 + 30 + 31 = 151 days for January through May, so it is day 151 + 15 = 166 of 365. That leaves 199 days remaining and puts the year 45.5% elapsed, in ISO week 25 (2026-W25-1, a Monday). Its codes are 26166 (YYDDD) and 2026166 (YYYYDDD), and its Julian Day Number is 2461207. Going the other way, day 100 of 2026 is April 10, while day 100 of the leap year 2028 is April 9.
Sources: ISO 8601 — Date and time format · U.S. Naval Observatory — Julian date converter
Frequently asked questions
Is this the same as a Julian date?
In everyday use, yes — food producers, the military and many log formats call the day-of-year (often as YYDDD) the “Julian date”. Astronomers use the Julian Day Number, a continuous count of days since 4713 BC; the calculator shows both, so pick the one your system expects.
How do I decode a code like 26166?
Choose “Decode a Julian code” and type it. The first two digits are the year (26 = 2026) and the last three are the day of the year (166 = June 15). Seven-digit codes such as 2026166 carry the full year.
How does a leap year change the count?
From 1 March onward, every day-of-year is one higher than in a common year, and the year has 366 days. 31 December is day 365 normally but day 366 in a leap year.
What is day 100 of the year?
In a common year it is 10 April; in a leap year it is 9 April, because 29 February comes first. Enter any day number and year to look up others.
Why can the ISO week belong to another year?
ISO weeks start on Monday and week 1 is the one containing the year’s first Thursday, so January 1–3 can fall in the last week of the previous year and December 29–31 in week 1 of the next.
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This calculator is for educational purposes. Double-check important results before acting on them.