paragraphs_gantt-1.0.x-dev/js/field_duration.js

(function ($, Drupal, once) {
  Drupal.behaviors.duration = {
    attach(context) {
      $(once('duration-start', 'input.start-date',context)).change(function () {
        let field_duration = $(this).data('field-duration');
        let parent = $(this).closest('.field--widget-duration-default');
        var startDate = new Date($(this).val());
        var endDate = parent.find('input.end-date');
        let selectorDuration = `input[name*=${field_duration}]`;
        if ($(this).val()) {
          if (endDate.val()) {
            let diffDays = dayBetween($(this).val(), endDate.val())
            $(selectorDuration).val(diffDays);
          } else if ($(selectorDuration).val()) {
            let end = calculateEndDate(startDate, $(selectorDuration).val());
            endDate.val(end);
          }
        }
        // Duration to end date.
        $(selectorDuration).change(function () {
          let duration = parseInt($(this).val());
          let end = calculateEndDate(startDate, duration);
          endDate.val(end);
        });
      });

      $(once('duration-end', 'input.end-date', context)).change(function () {
        let field_duration = $(this).data('field-duration');
        let selectorDuration = `input[name*=${field_duration}]`;
        let parent = $(this).closest('.field--widget-duration-default');
        let starDate = parent.find('input.start-date');
        if ($(this).val() && starDate.val()) {
          let diffDays = dayBetween(starDate.val(), $(this).val());
          $(selectorDuration).val(diffDays);
        }
      });

    }
  };

  function dayBetween(start, end) {
    start = new Date(start);
    end = new Date(end);
    let timeDiff = Math.abs(end.getTime() - start.getTime());
    return Math.ceil(timeDiff / (1000 * 3600 * 24));
  }

  function calculateEndDate(start, duration) {
    if (isNaN(duration)) {
      duration = 0;
    }
    let end = new Date(start.getTime() + (duration * 24 * 60 * 60 * 1000));
    let year = end.getFullYear();
    let month = ('0' + (end.getMonth() + 1)).slice(-2);
    let day = ('0' + end.getDate()).slice(-2);
    return year + '-' + month + '-' + day;
  }

}(jQuery, Drupal, once));