You want to find out what day Twelfth Night falls on after Christmas 2018. Which code should you use?
$xmas = new DateTime('Dec 25, 2018'); $twelfth_night = $xmas->add(new DateInterval('P12D')); echo $twelfth_night->format('l');
$xmas = new DateTime('Dec 25, 2018');
$twelfth_night = $xmas->add(new DateInterval('P12D'));
echo $twelfth_night->format('l');
$twelfth_night = strtotime('December 25, 2018 + 12 days'); echo date('d', $twelfth_night);
$twelfth_night = strtotime('December 25, 2018 + 12 days');
echo date('d', $twelfth_night);
$twelfth_night = strtotime('December 25, 2018 + 12 days'); echo strftime('%d', $twelfth_night);
echo strftime('%d', $twelfth_night);
$xmas = new DateTime('Dec 25, 2018'); $twelfth_night = $xmas->add(strtotime('12 days')); echo $twelfth_night->format('D'); 1 seems correct, but the question asks for "day", not day of the week. Twelfth Night is the "06" day of January, 2019.
$twelfth_night = $xmas->add(strtotime('12 days'));
echo $twelfth_night->format('D');