Using form_alter to change the default value of a date

If you need to set a default value for an exposed views filter, you can do it like this:

this example code will set a ranged date to have a default value of -30 days for the start date, and today for the end date

function hook_form_views_exposed_form_alter(&$form, $form_state, $form_id) {

  switch ($form_state['view']->name) {
  case 'my_view_name':
    // the Date object works only with this date format:
    $datemask = 'Y-m-d'; 
    $form['field_date_value']['min']['#default_value'] = date($datemask, time()-30*86400);
    $form['field_date_value']['max']['#default_value'] = date($datemask);
    break;

    $datemask = 'd/m/Y';
    $form['field_date_value']['min']['#date_format'] = $datemask;
    $form['field_date_value']['max']['#date_format'] = $datemask;
  }
}