Question

How do you go in php from a nth day in the year to the date like:

getdatefromday(275, 2012) 

and it outputs a date (better if an object).

And I’d like to do the opposite too, like getdayoftheyear("21 oct 2012")

Answer 1

This is pretty easy all around. You should read up on the DateTime object’s createFromFormat static method here, the date function here and the strtotime function here.

/**
 * Converts the day of year of a year into a DateTime object
 * @param int $year The year
 * @param int $dayOfYear The day of year (value 1 till 365 or 1 till 366 for leap years)
 * @return \DateTime The resulting date
 */
function getDateFromDay(int $year, int $dayOfYear): \DateTime {
    // Note: "Y z" and "z Y" make a difference for leap years (last tested with PHP 8.0.3)
    $date = \DateTime::createFromFormat('Y z', strval($year) . ' ' . strval($dayOfYear-1));
    return $date;
}