Remove fieldset on a date field Drupal 7

For some reason you want to display the date field without the fieldset wrapper, it’s not difficult but it may take you time to notice that the theme function called is in the date.theme file (date module) :

/**
 * Returns HTML for a start/end date combination on form.
 */
function theme_date_combo($variables) {
  $element = $variables['element'];
  $field = field_info_field($element['#field_name']);
  $instance = field_info_instance($element['#entity_type'], $element['#field_name'], $element['#bundle']);
 
  // Group start/end items together in fieldset.
  $fieldset = array(
    '#title' => t($element['#title']) . ' ' . ($element['#delta'] > 0 ? intval($element['#delta'] + 1) : ''),
    '#value' => '',
    '#description' => !empty($element['#fieldset_description']) ? $element['#fieldset_description'] : '',
    '#attributes' => array(),
    '#children' => $element['#children'],
  );
  return theme('fieldset', array('element' => $fieldset));
}

So to override the funtion and change the render of the field put this code in the template.php file of your theme :

/**
 * Render date field without fieldset
 */
function MYTHEME_date_combo($variables) {
  return theme('form_element', $variables);
}

Nice coding!