Description:
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When using date_create to find out the day of the year for a given date using the date string format 'z', you expect a result from 0-365.

When testing 2008-01-02 you get the expected result 1.
With 2008-01-01 the format returns null, incorrectly.

Reproduce code:
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$date_time = date_create('2008-01-01 00:00:00');
$formatted_time = $date_time->format('z');
var_dump($formatted_time);

Expected result:
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integer of value zero.

Actual result:
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null