Calculate a user-entered date/time difference

Parse both inputs and use DateTimeInterface::diff() for a calendar-aware interval.

$start = new DateTimeImmutable('2026-09-14 09:00');
$end = new DateTimeImmutable('2026-09-16 17:30');
$interval = $start->diff($end);
echo $interval->format('%a days %h hours %i minutes');

Current example first; the detailed examples and demos below are retained for additional learning and comparison.