Make valid_date() public

Is there any reason that valid_date() should be protected? It seems like it'd be useful to let another script find out if a given date falls on a recurring date.

`$r = new When();
		var_dump( $r->recur('20130104T090000')->rrule('FREQ=WEEKLY;UNTIL=20130216T003000Z;BYDAY=FR')->valid_date( new DateTime('yesterday') ) );`

If there's a better way to find this out, please let me know, thanks!