Description:
------------
"ordinal weeks" can be parsed, but "ordinal week" cannot.
That seems to be tried to parse as timezone name.

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

try {
    
    $results['first weeks'] = (new \DateTime('first weeks'))->format('Y-m-d');
    $results['first week'] = (new \DateTime('first week'))->format('Y-m-d');
    
} catch (\Exception $e) {
    
    $errors[] = $e->getMessage();
    
}

print_r(compact('results', 'errors'));

Expected result:
----------------
Array
(
    [results] => Array
        (
            [first weeks] => 2015-03-12
            [first week] => 2015-03-12
        )

)
[Finished in 0.5s]

Actual result:
--------------
Array
(
    [results] => Array
        (
            [first weeks] => 2015-03-12
        )

    [errors] => Array
        (
            [0] => DateTime::__construct(): Failed to parse time string (first week) at position 0 (f): The timezone could not be found in the database
        )

)