Is it possible to have placeholder text on a view filter with type date_text?
You are doing it wrong way

Form alter doesn't work here.
This is how it can be achieved:

It depends on which date element you are using
There are few hooks available

hook_date_text_process_alter()
hook_date_popup_process_alter()
hook_date_combo_process_alter()
hook_date_select_process_alter()


I am showing for hook_date_text_process_alter()

/**
 * Implements hook_date_text_process_alter().
 */
function MYMODULE_date_text_process_alter(&amp;$element, &amp;$form_state, $context) {
  if ($form_state['complete form']['#form_id'] == 'views_exposed_form') {
    //dpm($form_state);
    if ($element['#id'] == 'edit-field-appintment-date-value-value') {
      $element['date']['#attributes']['placeholder'] = t('Demo placeholder');
      //dpm($element);
    }
  }
}


For Other text elements
Below is the correct example of adding a placeholder to views_exposed_form to any other text field but date field can't be done like this.

/**
 * Implements hook_form_views_exposed_form_alter().
 */
function MYMODULE_form_views_exposed_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  $form['title']['#attributes'] = array('placeholder' =&gt; array(t('Your Placeholder')));
}


Hope this helps