code: function(dt) {
        if(!dt.isValid()) {
          if(this.options.errorClass) {
            this.$widget.addClass(this.options.errorClass);
          } else {
            //store original border color
            if(!this.borderColor) {
              this.borderColor = this.$widget.find('select').css('border-color'); 
            }
            this.$widget.find('select').css('border-color', 'red');
          }
        } else {
          if(this.options.errorClass) {
            this.$widget.removeClass(this.options.errorClass);
          } else {
            this.$widget.find('select').css('border-color', this.borderColor);
          }  
        }
      }
docs: /*
  highlight combos if date is invalid