Refactor mdate() function

[simobern]

function mdate($date = null) {
  $mdate = $date == null ? time() : strtotime($date);
  return is_a($date, 'MongoDB\BSON\UTCDateTime') ? $date : new MongoDB\BSON\UTCDateTime($mdate);
}

[iamdaniele]

See #4:

Would class_alias be a better alternative?

class_alias('MongoDB\BSON\UTCDateTime', 'MongoDate');