I have created a filter to modify the post date, using the method described at http://www.gravityhelp.com/documentation/page/Gform_post_data *
however in the latest version of GF I find that the changed date gets overriddden when the post is updated (see forms_model.php - line 2217)
In order to remedy it, I had to add
$post->edit_date = 'false'; at line 2215 so that changes to the date would not be affected.
please let me know if this will be remedied in the next update, either by adding the code I mention above, or by adding a filter to that new post object, prior to the post being updated.
Here is a screencast of the problem: http://screencast.com/t/FeFt3m4a
* Also, the location of Gform_post_data hook is incorrectly shown in the "Source Code" section of the documentation link above. It should read 'forms_model.php' *not* 'form_display.php'