Is it possible to implement date field as one select list?
I would recommend replacing the date field with text box and add a date pop to allow certain dates. This could be easily done by hook_form_alter in your custom module. See below:

    /**
     * Implements hook_form_alter().
     */
    function yourmodule_form_alter(&amp;$form, $form_state, $form_id) {
      switch($form_id) {
        case 'your_form_id':
          $form['yourfield']['#type'] = 'date_popup';
          $form['yourfield']['#date_format'] = 'd/m/Y';
          $form['yourfield']['#datepicker_options'] = array('minDate' =&gt; '-15D', 'maxDate' =&gt; '-1D');
          $form['yourfield']['#readonly'] = TRUE;
          break;
      }
    }


You will need the Date  module enabled before you can use this.
Let me know how you go..