Description:
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DatePeriod allows calculating the number of occurrences of a given interval in the specified period (from start to end).

For example, we can obtain the number of hours for each calendar day in the year with `iterator_count()` and print out the number if it is not 24.

Using the example code, we can determine that one hour is missing in 2017 when using this method of calculation and a timezone other than `UTC`.

Test script:
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$interval = new \DateInterval('PT1H'); // plus time one hour
$i = 0;
while ($i < 366) {
    $start = new \DateTime('2017-01-01 00:00:00', new \DateTimeZone('Europe/Berlin'));
    $start->add(new \DateInterval('P'.$i.'D'));

    $end = new \DateTime('2017-01-02 00:00:00', new \DateTimeZone('Europe/Berlin'));
    $end->add(new \DateInterval('P'.$i.'D'));

    $period = new \DatePeriod($start, $interval, $end);
    $diff = iterator_count($period);

    if ($diff != 24) {
        echo sprintf('%s, %d', $start->format(\DateTime::ISO8601), $diff);
    }
    $i++;
}


Expected result:
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We expect to have two values printed to console (i.e. there is one iterator with 23 counts and one with 25 counts):

2017-03-26T00:00:00+0100, 23
2017-10-29T00:00:00+0100, 25

The first, when daylight savings time (DST) starts, the second when DST ends.

This means that the total number of hours in 2017 is 8760 which is equivalent to 365 days of 24 hours.

Actual result:
--------------
Only one value is printed to console:

2017-03-26T00:00:00+0100, 23

This means that the total number of hours in 2017 is 8759 which is _not_ equivalent to 365 days of 24 hours. In other words, one hour is missing, even though 2017 is a "regular" year without leap days, leap seconds, etc..

In contrast, when using `UTC` as timezone in the example code, the year has 8760 hours as expected.

Double-checking with `DateTimeZone::getTransitions()` for `Europe/Berlin` shows that a transition on 2017-10-29 is correctly defined.