Day of the year calculator, what number is it?

The day of the year is the position of a date within its year: 1 January is day 1 and 31 December is day 365, or day 366 in a leap year. The calculator above shows today's number the moment it opens. As a worked example, 18 October 2026 is day 291 of 365, with 74 days left in the year, in ISO week 42. It also works the other way, from a day number to a date.

How to use the calculator

1. Read the big card. It is the Day of the year for the date in the Date box, which opens on today. Today puts it back.
2. Pick any other date in Date to see its number, the days left, the ISO week, the quarter, the leap year and the weekday.
3. To go the other way, type a Year and a Day number. The card under A day number as a date shows the date.
4. Press Copy result for one sentence of text, or Copy link for a link that opens on the same date.

Counting by hand

Add the days of all the months before the date, then add the day of the month. For 18 October, the nine months before it have 31, 28, 31, 30, 31, 30, 31, 31 and 30 days, which add up to 273. Then 273 plus 18 is 291. The table has the first and last day number of every month, for a common year and for a leap year.

The month ranges match the ordinal date table in a draft of ISO 8601 part 1 kept on the Library of Congress site (a 2016 working draft, ISO/WD 8601-1, read 18 Oct 2026).

Leap years and leap day

The same draft states the Gregorian rule: a leap year is a year divisible by four, except a centennial year, which is a leap year only when it is divisible by four hundred. So 2028 and 2000 are leap years, and 1900 and 2100 are not. In a leap year 29 February is day 60 and every date after it moves one number up: 1 March is day 60 in 2026 but day 61 in 2028, and 31 December is day 365 in 2026 but day 366 in 2028. Try 29 February 2028 in the Date box, then type 2026 and day 366 to see what happens when a day does not exist.

The ISO week number

The ISO week starts on Monday. The draft says the first week of a year is the one that includes the first Thursday, and a year has 52 or 53 weeks. That means the start of January can belong to the last week of the year before, and the end of December to week 1 of the next. 1 January 2027 is a Friday, so it is in week 53 of 2026, written 2026-W53-5. 30 December 2024 is a Monday in week 1 of 2025, written 2025-W01-1. The page shows the week date for any day. Calendars that start the week on Sunday, as many US ones do, can give another number.

Ordinal dates and the two Julian dates

The ordinal date is the year and the day number: 18 October 2026 is 2026-291. The same draft defines it as the year and a three digit day number, with 001 for 1 January, in the extended form YYYY-DDD and the basic form YYYYDDD. This is the ordinal date format of ISO 8601.

The words Julian date mean two different things. In some industries and systems, a Julian date is the year and day number written as YYDDD, so 18 October 2026 is 26291, or as YYYYDDD, 2026291. In astronomy, a Julian date is a continuous count of days. The US Naval Observatory's converter describes Julian dates as a continuous count of days and fractions since noon Universal Time on January 1, 4713 BC on the Julian calendar (checked 18 Oct 2026, USNO Julian Date Converter, read through an archived copy). At 00:00 UTC on 18 October 2026 that count is 2461331.5, and the Julian day number for the date is 2461332. Neither has anything to do with the Julian calendar of Julius Caesar.

Common problems

• Day 366 gives no date. Only leap years have a day 366. The card says so and names day 1 of the next year.
• The week number differs from my calendar. This page uses ISO weeks, which start on Monday and put week 1 around the first Thursday.
• Today is not what I expected. The page uses this device's date. Check the device's time zone.

To count the days between two dates, use days between dates. To find the days left to a date, use the countdown to date. For a Unix time, see the Unix timestamp converter.