When running unit test calendar/tests/rrule_manager_test.php

 function test_yearly_september_every_two_years_forever()

It uses the month of September, if you try and run the test on the 31st day of a month i.e. 31/05/17 it will try and change the event start date to 31/09 but as this day doesn't exist it changes it to 01/10 hence causing the test to fail.

Looks like 2 options to sort the issue, 

hardcode the day as 01or change the month to one that has 31 days.

Error

Moodle 3.2.3+ (Build: 20170525), 9651aba2fe0a6654ce1aa515fa89fd2d24748689

Php: 5.6.30.10.2, pgsql: 9.3.16, OS: Linux 3.19.0-25-generic x86_64

PHPUnit 5.5.7 by Sebastian Bergmann and contributors.

.......................................FDateTime Object

(

    [date] => 2017-10-01 09:00:00.000000

    [timezone_type] => 3

    [timezone] => US/Eastern

)

......................... 65 / 85 ( 76%)

....................                                              85 / 85 (100%)

Time: 9.72 seconds, Memory: 69.50MB

There was 1 failure:

1) core_calendar_rrule_manager_testcase::test_yearly_september_every_two_years_forever

Failed asserting that two strings are equal.

— Expected

+++ Actual

@@ @@

-'2017-10-01 09:00:00'

+'2019-09-01 09:00:00'

FIX - Change of month to May - 31 days.

  public function test_yearly_september_every_two_years_forever() {

        global $DB;

        // Change the start date for forever events to 9am on the month of MAY of the current year using the current day.

        $this->change_event_startdate(date('Y0*5*d\T090000'));

         $rrule = "FREQ=YEARLY;BYMONTH=5;INTERVAL=2"; // Forever event.

        $mang = new rrule_manager($rrule);

        $untildate = new DateTime();

        $untildate->add(new DateInterval('P' . $mang::TIME_UNLIMITED_YEARS . 'Y'));

        $untiltimestamp = $untildate->getTimestamp();

        $mang->parse_rrule();

        $mang->create_events($this->event);

        $records = $DB->get_records('event', ['repeatid' => $this->event->id], 'timestart ASC', 'id, repeatid, timestart');

        $interval = new DateInterval('P2Y');

        $expecteddate = new DateTime(date('Y0*5*d\T090000'));

        print_r($expecteddate);

        foreach ($records as $record) {

            $this->assertLessThanOrEqual($untiltimestamp, $record->timestart);

            $this->assertEquals($expecteddate->format('Y-m-d H:i:s'), date('Y-m-d H:i:s', $record->timestart));

            // Go to the next expected date.

            $expecteddate->add($interval);

        }

    }