[rgl]

Альтертанивный вариант (извиняюсь что без новомодных let и =>)

function getDiff(startDate, endDate) {
//  var date1 = new Date(startDate);
//  var date2 = new Date(endDate);
//  var diff = Math.abs(date2-date1);
  var diff = Math.abs(new Date(endDate) - new Date(startDate));

  diff = (diff-(diff%1000))/1000;
  var seconds = diff % 60;
  diff = (diff-seconds)/60;
  var minutes = diff % 60;
  diff = (diff-minutes)/60;
  var hours = diff % 24;
  var days = (diff-hours)/24;
  return days+"d "+hours+"h "+minutes+"m "+seconds+"s";
}