Altering the _form_alter & adding new fields

In my module I want to alter the search form by adding more extra fields to it, the problem is that I dont know where I should start working on the next step after I modify the form.

using drupal api module_form_alter($form_id , &$form)
I added my extra date field to the form, like following

function module_form_alter($form_id, &$form) {

      //date fields
$newform = array()
        $newform['advancedsearch']['from_date'] = array(
      '#title' => t('From Date'),
      '#type' => 'date',
      '#description' => t('Search From this Date.'),
      '#default_value' => array(
      'month' => format_date(time(), 'custom', 'n'),
      'day' => format_date(time(), 'custom', 'j'),
      'year' => format_date(time(), 'custom', 'Y')
      )
      );

$form = array_merge($form, $newform );

}

I assume that the next step is in the validate function then to the submit function.
What is the correct submit function that i should be calling inorder to handle the new date variable with the search action.