This looks for all mail labeled "To be archived" that is older than 1 day, archives it and removes the label.

ArchiveMail.gs

function archiveMail() {
  var delayDays = 1 // Enter # of days before messages are archived
  var maxDate = new Date();
  maxDate.setDate(maxDate.getDate()-delayDays);
   
  var label = GmailApp.getUserLabelByName("To be archived");
  if (label != null) {
    var threads = label.getThreads();
    for (var i = 0; i < threads.length; i++) {
      if (threads[i].getLastMessageDate()<maxDate) {
        threads[i].moveToArchive();
        threads[i].removeLabel(label);
      }
    }
  }
}