Taking data from date field and using it elsewhere in Wordpress

I enclose the code that I inserted. maybe I did incorrect modifications:

add_filter("gform_post_data", "change_post_date", 10, 3);
function change_post_date($post_data, $form, $entry){

//Only change post date on form id 124.
//NOTE: Replace 124 with your actual form id
if($form["id"] != 3)
return $post_data;

//NOTE: Replace 3 with your date field id. You can find your field id by inspecting the field's input tag
$date = $entry[27];

$post_data["post_date"] = date("Y-m-d", strtotime($date));
return $post_data;
}

ps: the date code is now 27 because I tried to create a custom post date field. It didn't work better than the date field

Alex I tried the modification but didn't work. there still might be a date format problem.

currently in my form the form number is 3, the date number is 28, the input date type is "date field" the date format is "dd/mm/yyyy"

so I inserted in the following text in function.php :

add_filter("gform_post_data", "change_post_date", 10, 3);
function change_post_date($post_data, $form, $entry){

//Only change post date on form id 124.
//NOTE: Replace 124 with your actual form id
if($form["id"] != 3)
return $post_data;

//NOTE: Replace 3 with your date field id. You can find your field id by inspecting the field's input tag
$post_data["post_date"] = $entry["28.3"] . "-" . $entry["28.2"] . "-" . $entry["28.1"];
return $post_data;
}

the result is that the posts are sent a today date. not working.

here is the code of the form for the date input:

<label class='gfield_label' for='input_3_28'>Date</label><div class='clear-multi'><div class='gfield_date_day ginput_container' id='input_3_28'><input type='text' maxlength='2' name='input_28[]' id='input_3_28.2' value='' tabindex='2' /><label for='input_3_28.2'>DD</label></div><div class='gfield_date_month ginput_container' id='input_3_28'><input type='text' maxlength='2' name='input_28[]' id='input_3_28.1' value='' tabindex='3' /><label for='input_3_28.1'>MM</label></div><div class='gfield_date_year ginput_container' id='input_3_28'><input type='text' maxlength='4' name='input_28[]' id='input_3_28.3' value='' tabindex='4' /><label for='input_3_28.3'>YYYY</label></div>

the other thing is that I will have several gravity forms which should send post with a user modified date.
so What would be the code for several forms and Date Input Type, date picker, calendar, dd/mm/yyyy?

hello Alex
I sent you the exported form by mail

still not working...
I tried to import the form in another blog with another theme (twentyten) and it still didn't work. the date is 1970.

I modified the "functions.php" of the theme file and you wrote "function.php".
I am wrong here?
I pasted your code at the end of this functions.php text.

it's very frustrating...

I can give you by mail the admin code eventually. something is wrong somewhere...

Alex, I have Discovered a critical problem : The date of a post (timestamp)
cannot be set before 1970 in wordpress.
By consequence using the timestamp date field to enter historical events in the simile timeline is not The right solution there.
In the simile Timeline wordpress plugin, a widget in the posting page enable to enter dates (start, end...).
But these fields are not custom type fields, they are recording data into custom database tables.
Do you think it could be possible to write data directly into database fields from gravity?

info on the wp simile timeline plugin here:
Simile timeline plugin : http://wordpress.org/extend/plugins/wp-simile-timeline/
http://groups.google.com/group/wp-simile-timeline