The error you report about the invalid DateTime is very similar to what was reported here: https://bugs.php.net/bug.php?id=72011
In the bug report two items were mentioned which may lead to a solution for you:
1) If APC or any other bytecode caching is in place on your server it may help to deactivate it
2) The bug occurs when the max_execution_time is reached while Garbage Collection is running. This would happen when the server is under heavy load. A possible solution would be to bump up the max_execution_time setting in the PHP configuration.