This is related to how Zend_Date returns year for YYYY & yyyy format. Detailed explanation is here http://framework.zend.com/issues/browse/ZF-5297
Sample code (adapted the Datetimefield setValue() method) to highlight the problem:
include 'framework/thirdparty/Zend/Date.php';
$userValueObj = new Zend_Date(null, null, 'en_US');
$userValueObj->setTimezone('GMT');
$userValueObj->setDate('2012-01-01', 'YYYY-MM-dd');
$userValueObj->setTime('00:00:00', 'HH:mm:ss');
echo $userValueObj->get('YYYY-MM-dd HH:mm:ss', 'en_US'), "\n"; // returns 2011-01-01 00:00:00
echo $userValueObj->get('yyyy-MM-dd HH:mm:ss', 'en_US'), "\n"; // returns 2012-01-01 00:00:00
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