When\When::freq PHP Method

    public function freq($frequency)
    {
        if (Valid::freq($frequency)) {
            $this->freq = strtolower($frequency);
            return $this;
        }
        throw new \InvalidArgumentException("freq: Accepts " . rtrim(implode(Valid::$frequencies, ", "), ","));
    }

 /**
  * Every Sunday in January every other year at 8:30 am and 9:30 am
  * DTSTART;TZID=America/New_York:19970105T083000
  * RRULE:FREQ=YEARLY;INTERVAL=2;BYMONTH=1;BYDAY=SU;BYHOUR=8,9;BYMINUTE=30
  */
 function testYearlyTheninthnode()
 {
     $results[] = new DateTime("2015-09-17 00:00:00");
     $results[] = new DateTime("2016-09-17 00:00:00");
     $results[] = new DateTime("2017-09-17 00:00:00");
     $results[] = new DateTime("2018-09-17 00:00:00");
     $results[] = new DateTime("2019-09-17 00:00:00");
     $r = new When();
     $r->startDate(new DateTime('2015-09-17 00:00:00'));
     $r->freq('yearly');
     $r->interval(1);
     $r->count(5);
     $r->generateOccurrences();
     $occurrences = $r->occurrences;
     foreach ($results as $key => $result) {
         $this->assertEquals($result, $occurrences[$key]);
     }
 }