Use bindValue() instead of passing parameters in execute().
Use DateTime object instead of fixed format date string.

Use a DateTime object and have conversion handled by Doctrine instead of using
a date string with a fixed format.
Use the Doctrine to convert date string to DateTime object.
Timestamp test is no longer necessary as we rely on Doctrine and DateTime.
Merge pull request #6052 from owncloud/datetime-doctrine

Get rid of date strings for DB. Completely use Doctrine and DateTime.