Description:
------------
For lots of negative-year dates, the day of the week is reported as "Unknown". 
Sundays (0th day) are okay. The most modern date affected appears to be -0007-10-
01.

The test script below runs through each day in the year -10 as an example.

Test script:
---------------
<?php

$utc = new DateTimeZone('UTC');
$period = new DatePeriod(
    new DateTime('-0010-01-01', $utc),
    new DateInterval('P1D'),
    new DateTime('-0009-01-01', $utc)
);

foreach ($period as $date) {
    if ($date->format('D') === 'Unknown') {
        echo $date->format('Y-m-d D') . PHP_EOL;
    }
}

?>

Expected result:
----------------
No output is expected.

Actual result:
--------------
-0010-01-01 Unknown
-0010-01-02 Unknown
-0010-01-03 Unknown
-0010-01-04 Unknown
-0010-01-05 Unknown
-0010-02-01 Unknown
-0010-02-02 Unknown
-0010-03-01 Unknown
-0010-03-02 Unknown
-0010-05-01 Unknown
-0010-05-02 Unknown
-0010-05-03 Unknown
-0010-05-04 Unknown
-0010-06-01 Unknown
-0010-08-01 Unknown
-0010-08-02 Unknown
-0010-08-03 Unknown
-0010-10-01 Unknown
-0010-10-02 Unknown
-0010-10-03 Unknown
-0010-10-04 Unknown
-0010-10-05 Unknown
-0010-11-01 Unknown
-0010-11-02 Unknown