Date fields (as well as time and grid fields) are not available when using webform_alt_ui - I found this bit of code:

  // Disable options that don't work with form_builder
  $form['components']['grid']['#printed'] = TRUE;
  $form['components']['grid']['#default_value'] = FALSE;
  $form['components']['date']['#printed'] = TRUE;
  $form['components']['date']['#default_value'] = FALSE;
  $form['components']['time']['#printed'] = TRUE;
  $form['components']['time']['#default_value'] = FALSE;

form_builder has added support for the date/time/grid components, so it'd be nice to get it in here. I made a stab at making some obvious changes (like removing the above;) but the components didn't show up.