/**
  * get_valid_timezone_string
  *
  * 	ensures that a valid timezone string is returned
  *
  * @access protected
  * @param string $timezone_string
  * @return string
  * @throws \EE_Error
  */
 public static function get_valid_timezone_string($timezone_string = '')
 {
     // if passed a value, then use that, else get WP option
     $timezone_string = !empty($timezone_string) ? $timezone_string : get_option('timezone_string');
     // value from above exists, use that, else get timezone string from gmt_offset
     $timezone_string = !empty($timezone_string) ? $timezone_string : EEH_DTT_Helper::get_timezone_string_from_gmt_offset();
     EEH_DTT_Helper::validate_timezone($timezone_string);
     return $timezone_string;
 }