gmailAutoarchive.js

function gmailAutoarchive() {
  
  var delayDays = 2; // will only impact emails more than 48h old
  
  var maxDate = new Date((new Date).getTime() - delayDays * 24 * 3600 * 1000);

  // Get all the threads labelled 'autoarchive'
  var label = GmailApp.getUserLabelByName("autoarchive");
  var threads = label.getThreads(0, 400);
  
  // we archive all the threads if they're unread AND older than the limit we set in delayDays
  for (var i = 0; i < threads.length; i++) {
    if (threads[i].getLastMessageDate().getTime() < maxDate.getTime())
    {
      threads[i].moveToArchive();
    }
  }
}