/**
  *		This function is a singleton method used to instantiate the Espresso_model object
  *
  *		@access public
  *		@param string $timezone string representing the timezone we want to set for returned Date Time Strings (and any incoming timezone data that gets saved).  Note this just sends the timezone info to the date time model field objects.  Default is NULL (and will be assumed using the set timezone in the 'timezone_string' wp option)
  *		@return EEM_Registration instance
  */
 public static function instance($timezone = NULL)
 {
     // check if instance of Espresso_model already exists
     if (self::$_instance === NULL) {
         // instantiate Espresso_model
         self::$_instance = new self($timezone);
     }
     //we might have a timezone set, let set_timezone decide what to do with it
     self::$_instance->set_timezone($timezone);
     // Espresso_model object
     return self::$_instance;
 }