Day of Year Calculator
Date to Day of Year
Convert a date to its day-of-year number (1-365/366)
Day Number to Date
Convert a day-of-year number back to a calendar date
Formula
DOY = Σ days_in_months(1..m-1) + day | Week = ⌈DOY / 7⌉
Frequently Asked Questions
What is the day of year?
The day of year (also called ordinal date or Julian day number) is a number from 1 to 365 (or 366 in a leap year) representing the sequential day count. January 1 is day 1, February 1 is day 32, etc.
How do I calculate week number from day of year?
Divide the day of year by 7 and round up. For example, day 50 is in week ceil(50/7) = week 8. Note that ISO week numbering may differ slightly.
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