Day-of-year numbers (sometimes wrongly called Julian dates) are used in manufacturing date codes, food packaging, aviation, the military and science. Day 256 is Programmers’ Day; day 100 marks the 100th day of the year.
How to use the Day of the Year Calculator
- Date
- Year (for day number → date)
- Day number
Results update as you type, and every field accepts the units shown in its menu.
How it works
Pick a date, or enter a year and day number to convert back.
Tips
- Check whether the end date should be included – event durations usually include it, elapsed time does not.
- Months have 28–31 days, so “one month” is counted by calendar date, not as 30 days.
- Use 24-hour time or set AM/PM carefully when entering clock times.
- Time zones and daylight-saving changes are not applied unless the calculator says so.
Common mistakes to avoid
- Counting a 30-day month when a calendar month is meant (or the reverse).
- Forgetting leap years when counting days across February.
- Mixing decimal hours (7.5 h) with clock time (7:30).
Inputs explained
| Field | What to enter |
|---|---|
| Date | A date |
| Year (for day number → date) | A number |
| Day number | A number |
What each result means
- Day of the year = dayofyear([Date])
- Days in the year = 365 + isleap(year([Date]))
- Days left in the year = [Days in the year] – [Day of the year]
- Year completed = [Day of the year]÷[Days in the year]×100
- ISO week number = (floor((dayofyear(([Date]) – mod(weekday([Date]) + 6, 7) + 3) – 1)÷7) + 1)
- Date of day number = date([Year (for day number → date)], 1, 1) + [Day number] – 1
Example values
How the main result changes when Year changes and everything else stays at the example values above:
| Year (for day number → date) | Day of the year |
|---|---|
| 1,013 | 298 |
| 1,519.5 | 298 |
| 2,026 | 298 |
| 2,532.5 | 298 |
| 3,039 | 298 |
| 4,052 | 298 |
Formula
Day of year = date − 31 December of the previous yearDays left = days in year − day of yearDate from day number = 1 January + (day number − 1)Worked example
Inputs — Date: 2026-10-25; Year (for day number → date): 2,026; Day number: 256.
Results — Day of the year = 298; Days in the year = 365; Days left in the year = 67; Year completed = 81.6 %; ISO week number = 43; Date of day number = Sun, September 13, 2026.
Frequently asked questions
What day of the year is it?
See the result – 1 January is day 1 and 31 December is day 365 (366 in a leap year).
What is a Julian date code?
In manufacturing, a code like 26298 often means year 2026, day 298.
What is day 256?
13 September (12 September in leap years) – Programmers' Day.
What formula does the Day of the Year Calculator use?
Day of year = date − 31 December of the previous year.