How can I change the selectable years in a date field?
@mpdonadio♦'s answer provides the explanation.  Here is the actual code I used: 

function mymodule_form_user_register_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  $form['field_birthday']['widget'][0]['value']['#date_year_range'] = '1950:2005';