Day of the Year Calculator
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Leap Year
About Day Numbers
The Day of the Year (DOY) is a number between 1 and 365 (or 366 in a leap year), representing the sequential day since January 1st.
$$ \text{DOY} = (\text{Date} - \text{Jan 1}) + 1 $$
Leap Year Rule
A year is a leap year if it is divisible by 4, except for end-of-century years, which must be divisible by 400.