auto-archive.gs

function autoArchive() {
  var delayDays = 7; // 7 days before messages are moved to archive.
  
  var maxDate = new Date();
  maxDate.setDate(maxDate.getDate() - delayDays);
  
  var threads = GmailApp.search("in:inbox -is:starred")
  threads.forEach(function (thread) {
    if (thread.getLastMessageDate() < maxDate) {
      thread.moveToArchive()
    }
  });
}