1. What is Day of Year?

Day of Year (DOY) is a numerical representation of the day within a given year, ranging from 1 through 365 (or 366 in leap years). January 1 is day 1, and December 31 is day 365 or 366.

2. How Does the Calculator Work?

The calculator uses the following formula:

Where:

• \( date \) — The input date
• \( jan1 \) — January 1st of the same year as the input date
• \( doy \) — The resulting day of year (1-366)

Explanation: The calculator subtracts January 1st from the input date to get the number of days between them, then adds 1 to make January 1st equal to day 1.

3. Importance of Day of Year

Details: Day of Year is commonly used in scientific, computing, and data analysis applications where a simple numerical representation of dates is needed. It's particularly useful for time series analysis and simplifying date calculations.

4. Using the Calculator

Tips: Simply enter any date in the format YYYY-MM-DD and click calculate. The calculator will determine the day of year for that date.

5. Frequently Asked Questions (FAQ)

Q1: How does the calculator handle leap years?
A: The calculator automatically accounts for leap years. February 29th will be correctly calculated as day 60 in leap years.

Q2: What's the range of valid dates?
A: The calculator works for any date from year 1 to year 9999, as supported by PHP's DateTime class.

Q3: Can I use this for dates before 1582 (Gregorian calendar reform)?
A: While the calculator will give a result, be aware that dates before 1582 may not accurately reflect historical calendar systems.

Q4: How is day of year different from Julian date?
A: Day of year is simpler (1-366) while Julian date is a continuous count of days since January 1, 4713 BCE.

Q5: What are common uses for day of year?
A: Common uses include astronomy, meteorology, file naming conventions, and simplifying date calculations in programming.