Implements hook_field_instance_settings_form().
function datetime_field_instance_settings_form($field, $instance) {
$settings = $instance['settings'];
$form['default_value'] = array(
'#type' => 'select',
'#title' => t('Default date'),
'#description' => t('Set a default value for this date.'),
'#default_value' => $settings['default_value'],
'#options' => array(
'blank' => t('No default value'),
'now' => t('The current date'),
),
'#weight' => 1,
);
return $form;
}