Calculate number of days between two date fields

custom.js

jQuery(function ($) {
    var $startDate = $('.quform-field-1_3'),
        $endDate = $('.quform-field-1_4'),
        $hiddenField = $('.quform-field-1_5'),
        datePicker = $startDate.data('kendoDatePicker'),
        options = $startDate.data('options') || {},
        culture = options.locale || 'en-US',
        format;

    if (window.kendo && datePicker) {
        format = options.format ? options.format : kendo.getCulture(options.locale).calendars.standard.patterns.d;

        $startDate.add($endDate).on('keyup blur', onDateChange);
        datePicker.bind('change', onDateChange);
    }

    function onDateChange() {
        var start = $startDate.val(),
            end = $endDate.val(),
            days = '';

        if (Quform.isNonEmptyString(start) && Quform.isNonEmptyString(end)) {
            var startDate = kendo.parseDate(start, format, culture),
                endDate = kendo.parseDate(end, format, culture);

            if (startDate !== null && endDate !== null) {
                days = daysBetweenTwoDates(startDate, endDate);
            }
        }

        $hiddenField.val(days).triggerHandler('change');
    }

    function daysBetweenTwoDates(startDate, endDate) {
        var fromDateUTC = Date.UTC(startDate.getFullYear(), startDate.getMonth(), startDate.getDate());
        var endDateUTC = Date.UTC(endDate.getFullYear(), endDate.getMonth(), endDate.getDate());

        return Math.ceil((+endDateUTC - +fromDateUTC) / kendo.date.MS_PER_DAY);
    }
});

The code can be added to Forms → Settings → Custom CSS & JS → Custom JavaScript.

• On line 2 replace 1_3 with the unique ID of the Start Date field
• On line 3 replace 1_4 with the unique ID of the End Date field
• On line 4 replace 1_5 with the unique ID of the field to store the number of days (it can be a Hidden field or Text field)