title: Zend_Date::isDate won't enforce $format
body: <p>I've having an issue where Zend_Date::isDate is returning true even when the value doesn't necessarily match the format given.</p>
  
  <p>For example:</p>
  
  <pre><code>$time = "12:34:56"; // Time doesn't have AM/PM attached
  
  if( Zend_Date::isDate($time, 'hh:mm:ss a') )
      echo "this is true";
  else
      echo "this is false";
  </code></pre>
  
  <p>This is always true, even though the format lists the 'a' indicating it needs "Time of day, localized" (which in my case is "AM"/"PM"). Even a completely mis-formatted time, like '12:12:34:56:56' will still return true. A time of ':34:56' will return false however. Is this a bug or am I missing something in thinking the format I give it is what needs to be matched?</p>
  
  <p>Thanks!</p>