WhenQuarterly.php

    /**                                                                                                                         
     * Every three months (quarterly) on the first Monday of the month,                                                         
     * starting January 7 2019, until February 2, 2021 (issue #TODO)                                                            
     * DTSTART;TZID=America/New_York:20190107T170000                                                                            
     * RRULE:FREQ=MONTHLY;INTERVAL=3;BYDAY=1MO;WKST=MO;UNTIL=2021-02-01T18:00:00-0500                                           
     */
    function testQuarterlyOne()
    {
        $tz = new DateTimeZone("America/New_York");

        $results[] = new DateTime('2019-01-07 17:00:00', $tz);
        $results[] = new DateTime('2019-04-01 17:00:00', $tz);
        $results[] = new DateTime('2019-07-01 17:00:00', $tz);
        $results[] = new DateTime('2019-10-07 17:00:00', $tz);
        $results[] = new DateTime('2020-01-06 17:00:00', $tz);
        $results[] = new DateTime('2020-04-06 17:00:00', $tz);
        $results[] = new DateTime('2020-07-06 17:00:00', $tz);
        $results[] = new DateTime('2020-10-05 17:00:00', $tz);
        $results[] = new DateTime('2021-01-04 17:00:00', $tz);

        $r = new When();
        $r->startDate(new DateTime("20190107T170000", $tz))
          ->freq("monthly")
          ->interval(3)
          ->byday('1MO')
          ->until(new DateTime("2021-02-01T18:00:00", $tz))
          ->generateOccurrences();

        $occurrences = $r->occurrences;

        foreach ($results as $key => $result)
        {
            $this->assertEquals($result, $occurrences[$key]);
        }

        /* Check that $occurrences doesn't have extra results, as well */
        $this->assertEquals(count($occurrences), count($results));
    }