Implements hook_field_settings_form().
function datetime_field_settings_form($field, $instance, $has_data) {
$settings = $field['settings'];
$form['datetime_type'] = array(
'#type' => 'select',
'#title' => t('Date type'),
'#description' => t('Choose the type of date to create.'),
'#default_value' => $settings['datetime_type'],
'#options' => array(
'datetime' => t('Date and time'),
'date' => t('Date only'),
),
);
return $form;
}