Select variant according to number of days

booking-helper.v2.js

var selectVariant = function(form) {
  var start = form.getStartDate(),
  	  finish = form.getFinishDate();

  if (start && finish) {
    var quantity = bta.dayDelta(start, finish), minutes = (quantity * 1440);
    $('#quantity').val(quantity);

    var index = -1, low = 0,
        configs = bta.parseVariantConfigs(form.datepicker('start').attr('data-bta-variant-config'));

    for (var id in configs) {
      var config = configs[id],
  		  duration = form.getDuration();

	  if (minutes > low) {
          index += 1;
      }

      low = duration;
    }
    
    // do stuff here to update the currently selected variant
    
    // this is for the 'startup' theme which uses Backbone
    $('#product-select option').removeAttr('selected');
    $('#product-select option:eq(' + index + ')').attr('selected', 'selected');
  
    $('#product-select-option-0 option').removeAttr('selected');
    $('#product-select-option-0 option:eq(' + index + ')').attr('selected', 'selected');
  
    $('#product-select-option-0').trigger('change');
  }
};

$('form[action="/cart/add"]').on('bta.datetimeChange', function(event, form) {
  selectVariant(form);
});