Here's a simple example on how this class works :
// Create a DateTimeImmutable Object
$date = new DateTimeImmutable('2000-01-01');
// "Change" that object and assign it's value to a new variable
$date2 = $date->add(new DateInterval('P6M5DT24H'));
// Check out the content of the two variables
echo $date->format('Y-m-d') . "\n";
// 2000-01-01
echo $date2->format('Y-m-d') . "\n";
// 2000-07-07