gmailAutoarchiveReaded.js

//version with unread check and nested labels
function gmailAutoarchive() {

  var delayDays = 1; // will only impact emails more than 24h old
  var maxDate = new Date();
  maxDate.setDate(maxDate.getDate()-delayDays); // what was the date at that time?

// Get all the threads labelled 'label_child2'
  var threads = GmailApp.search("in:inbox label:label_parent-label_child1-label_child2");

// 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()<maxDate && threads[i].isUnread()==false)
        {
          threads[i].moveToArchive();
        }
      }
}