I think you might be able to push a created_at value in a push or a post.
I make a request to update several fields for the test.
'created_at' => '2013-05-04T11:49:06.782Z',
'user_fields' => [
1 => '104',
2 => 'Patron'
]
In this case User_fileds are updated without any problems. So I decided to ask in the forum. Maybe created_date is updated by some other request such as trust_levels which have a special url API.