public function testAlter()
 {
     $datum = new Datum('1977-09-18');
     $this->assertEquals(34, $datum->Alter(new Datum('2011-01-01')));
     $this->assertEquals(34, $datum->Alter(new Datum('2011-12-31')));
     $this->assertEquals(1, Datum::Today()->Alter(Datum::Today()->DateAdd(365)));
 }