You should use the value submitted by the user which is on the $form_state
;
I tested with a single date field, instead of a datefield with 2 values. But this should work just fine anyways. I might have the naming of your field incorrect, but you can check out the values on $form_state['values']
, where you should find your field listed.
Anyways the code would look something like this:
function billing_submit_function($form, &$form_state) {
$from_date = $form_state['values']['field_bill_start'][0]['value'];
$from_date_string = format_date(strtotime($from_date), 'custom', 'D d/m/Y');
$from_date_2 = $form_state['values']['field_bill_start'][0]['value2'];
$from_date_string_2 = format_date(strtotime($from_date_2), 'custom', 'D d/m/Y');
drupal_set_message("From Date = ".$from_date_string);
drupal_set_message("From Date2 = ".$from_date_string_2);
}