code: function date(event) {
      var result = this.lib.expiry(this.month.get('value'), this.year.get('value'))
        , className = result ? 'valid' : 'invalid';
  
      //
      // Update both select boxes.
      //
      this.month.removeClass('invalid').addClass(className);
      this.year.removeClass('invalid').addClass(className);
    }
docs: On date changes, year or month, check expiration date.
  
  @param {Event} event
  @api private