What day of the year is it (out of 365 or 366)? Plus ISO 8601 week, quarter, days remaining. Date ↔ day number both ways.
The day-of-year (also called ordinal day or year day number) is just a count: January 1 is day 1, December 31 is day 365 — or day 366 in a leap year. The tool opens with today's date pre-filled so the first thing you see is "today is day N of YYYY" with the percentage progress through the year. Change the date to any other day to see its number.
Normal years have 365 days. Leap years have 366 because February 29 is added. The rule:
So the answer to "what day of the year is it out of 365" is wrong if you ask it on a leap year — the right denominator is 366. The tool handles this automatically and shows the correct max for the selected year.
Less common but useful for scientific data and military date formats. Switch to Number → date, enter the year and the day-of-year number (1-366), and the tool returns the calendar date with ISO week and quarter. Useful when you receive data labeled 2026-153 (DDD-YYYY format) and need to know what calendar date that actually is.
Different from "week of the year" the way Excel might calculate it. ISO weeks:
Want to see how much of the year is left? "85 % through 2026" is a more visceral framing than "day 310 of 365" — both shown side by side.
Date math only — nothing leaves the browser.