How this calculator works
The day number is the count of calendar days from January 1 of the same year through the chosen date, so January 1 is day 1 and December 31 is day 365, or day 366 in a leap year. The days remaining count the days after the chosen date to the end of the year. The ISO week is shown separately because ISO weeks can begin in the previous year.
Formula
day of year = O(date) - O(January 1 of the same year) + 1. days remaining = 365 or 366 - day of year. Share of the year = day of year / 365 or 366. Leap years follow the rule: divisible by 4, except centuries not divisible by 400.
Worked example
2026-11-01 is day 305 of 365 in a non-leap year, leaves 60 days after it, falls in ISO week 44 and is 83.56 percent of the way through the year. January 1 is day 1 and December 31 is day 365.
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Frequently asked questions
Is January 1 day 0 or day 1?
This page uses day 1, the common ordinal convention. Some computing systems count from zero. If you are matching a program, check whether its day-of-year value starts at 0 or 1.
What is the difference between day of the year and ISO week?
Day of the year counts days inside one calendar year. An ISO week is a seven-day block that can start in December of the previous year. The two numbers move at different rates and are not simple multiples.
Why are there 366 days in a leap year?
February 29 is added. A year is a leap year when it is divisible by 4, unless it is a century year not divisible by 400. So 2000 was a leap year but 1900 was not.
How is the percentage calculated?
It divides the day number by the length of the year. It is not the percentage of the date and time elapsed, because no clock time is part of the input.