How to use the `Date` Object and the `DateInterval()`Function Together

How to add a DateInterval object to a DateTime object

    
     $checkout_length = new DateInterval('P3W');
 
     $due_date = new DateTime(); 

     $due_date->add($checkout_length);

How to subtract a DateInterval object from a DateTime object

    $voting_age = new DateInterval('P18Y');

    $dob = new DateTime(); $dob->sub($voting_age);

    echo("<p>You can vote if you were born on or before ' . $dob->format('n/j/Y') . "</p>\n");
    sub($voting_age);
    echo("You can vote if you were born on or before " . $dob->format('n/j/Y') . "?p>\n");
    ?>
    
How to determine the amount of time between two dates
    $now = new DateTime('2020-09-13 12:45:00');

    $due = new DateTime('2021-04 last day of midnight');

    $time_span = $now->diff($due);

    echo("<p>"Time between " . $now->format('m/d/Y h:m') . " and " . $due->format('m/d/Y h:m') . " is: " . $time_span->format('%R%dd %H:%I:%Sh') . "</p>\n"); // '-15d 12:45:00h'
    diff($due);
    echo("Time between " . $now->format('m/d/Y h:m') . " and " . $due->format('m/d/Y h:m') . " is: " . $time_span->format('%R%dd %H:%I:%Sh') . "
\n"); // '-15d 12:45:00h'
    ?>
    You can use a DateInterval object to represent a time span called a date interval.
To display a date part with a leading zero, change its format code to uppercase.