function GmailAutoarchive() {
  // No need for a label
  var delayDays = 21; // three weeks
  var maxDate = new Date();
  maxDate.setDate(maxDate.getDate()-delayDays); // what was the date at that time?

  var threads = GmailApp.search("in:inbox" + " older_than:" + delayDays.toString() + "d");
  
  for (var i = 0; i < threads.length; i++) {
    if (threads[i].getLastMessageDate()<maxDate)
    {
      threads[i].moveToArchive();
    }
  }
}