Day number
N = C(m) + d + B
B is 1 when the year is a leap year and m > 2, otherwise 0.
Day of Year Calculator
See whether a date is the 1st, 100th or 282nd day of the year, how many days are left until 31 December and which quarter it belongs to.
Enter a valid date (day, month and a 4-digit year).
Day number
days
(year )
- Selected date
- Day of week
- Days remaining
- days
- Quarter
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How does this tool work?
9 October 2026 is the 282nd day of the year, with 83 days left until 31 December. This number, called the ordinal day or day of year, is used to date manufacturing batches, track a daily goal, write an ISO 8601 ordinal date (2026-282) or simply see how far into the year you are. Enter a date and the tool shows its number from 1 to 365 (366 in leap years), the days remaining, the weekday and the quarter, and tells you whether the year is a leap year.
The day number is the count of days since 1 January, that day included. It is obtained by adding the lengths of the full months already elapsed, then the day of the month. From 1 March onwards, a leap year adds 1 because 29 February has been inserted.
The tool applies the Gregorian leap-year rule: a year divisible by 4 is a leap year, unless it is divisible by 100 but not by 400. 2024 and 2000 therefore have 366 days, 1900 and 2100 only 365.
Remaining days do not include the date entered: 31 December shows 0. The quarter follows from the month (Q1 January–March, Q2 April–June, Q3 July–September, Q4 October–December). The date is handled without a time or time zone, so the result does not depend on where you are.
How to use this tool
Enter the date
Type the date in month/day/year order or pick it in the calendar.
Read the day number
The large figure is the ordinal day, followed by the total number of days in the year (365 or 366).
Use the extra details
Days remaining, weekday and quarter are shown alongside, with a note saying whether the year is a leap year.
How the day number is calculated
C(m) is the number of days in the full months before month m in a common year: 0 (January), 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334 (December).
Days remaining
R = L − N
L is 366 in a leap year, 365 otherwise.
Leap year
(Y mod 4 = 0 and Y mod 100 ≠ 0) or Y mod 400 = 0
- N
- day number within the year
- C(m)
- cumulative days before month m
- d
- day of the month
- B
- leap-day correction (0 or 1)
- L
- length of the year in days
- Y
- year
Example: 1 March 2024 → C(3) = 59, d = 1, B = 1, so N = 61.
Use cases
Batch codes and traceability
Many manufacturers encode the production date as year + day number (for example 26282). The tool gives the day number from a calendar date.
ISO 8601 ordinal dates
Write a date as YYYY-DDD (2026-282), a format used in some data exchanges, technical logs and file names.
Daily challenges
Reading, training or a “one step a day” savings plan: the day number shows your exact progress through the year.
Year-end deadlines
Days remaining tell you how much time is left before an annual closing or a goal set for 31 December.
Examples
An autumn date
273 days (January to September) + 9 = 282; 2026 is not a leap year, so 365 − 282 = 83.
Input
10/09/2026Result
Day 282 · 83 days left · Q41 March
In the leap year 2024, 29 February shifts every later date by one day: 1 March 2023 was only day 60.
Input
03/01/2024Result
Day 61 (366 days in the year)Programmers’ Day
Day 256 (2⁸) falls on 13 September in a common year and on 12 September in a leap year.
Input
09/13/2026Result
Day 256Tips and best practices
- The 100th day of the year is 10 April, or 9 April in a leap year.
- 2 July is the exact middle of a common year: 182 days before it, 182 days after it.
- In manufacturing and logistics, “Julian date” often means this day number (YYDDD format). Do not confuse it with the astronomers’ Julian Day, a continuous count since the year −4712.
- To count the days between any two dates, use the date difference calculator; for the week number, the ISO week calculator.
Sources and references
- ISO — ISO 8601 date and time format (opens in a new tab)Page in English
- US Naval Observatory — Introduction to calendars (opens in a new tab)Page in English
- Claus Tøndering, The Gregorian Calendar (opens in a new tab), Calendar FAQPage in English
- Julian Date Converter (opens in a new tab), U.S. Naval ObservatoryPage in English
Frequently asked questions
What day of the year is it today?
Enter today’s date: the number shown is the day of year. For example, 9 October 2026 is day 282.
Is 1 January day 0 or day 1?
Day 1. ISO 8601 ordinal numbering runs from 001 on 1 January to 365 or 366 on 31 December.
How do I know if a year is a leap year?
It is a leap year if it is divisible by 4, except century years not divisible by 400. 2028 will be a leap year, 2100 will not. The tool states it under the result.
Why does the number differ by one depending on the year?
From 1 March onwards, every date in a leap year has a number one higher because of 29 February. Before 1 March, the numbers are identical.
How is this different from the Julian Day?
The day of year restarts at 1 every 1 January. The astronomical Julian Day is a continuous counter (9 October 2026 at noon UT is Julian Day 2,461,323): they are two different quantities.
Do the remaining days include the date entered?
No. They count the days after the date up to and including 31 December: 282 + 83 = 365.
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