Thank you for taking the time to write to us, but this is not
a bug. Please double-check the documentation available at
http://www.php.net/manual/ and the instructions on how to report
a bug at http://bugs.php.net/how-to-report.php

This is correct. PHP automatically adjust for value underflows and overflows. Since more recent PHP versions, it will add a warning for invalid dates though:

<?php
$date = date_create('2011-02-31');
var_dump(date_get_last_errors());
?>

array(4) {
  ["warning_count"]=>
  int(1)
  ["warnings"]=>
  array(1) {
    [11]=>
    string(27) "The parsed date was invalid"
  }
  ["error_count"]=>
  int(0)
  ["errors"]=>
  array(0) {
  }
}