gmail_schedule_delete.gs

function gmail_schedule_delete() {
  var current = new Date();
  var onedaymilli = 24 * 60 * 60 * 1000;
  Logger.log("Current date: " + current);

  var label_common = "(schedule delete)/after ";
  var sublabels = [["1 week", 7],
                   ["2 weeks", 14],
                   ["1 month", 30], 
                   ["3 months", 90],
                   ["6 months", 180],
                   ["1 year" ,365]];

  for (var i = 0; i < sublabels.length; i++) {
    var label_name = label_common + sublabels[i][0];
    var label_days = sublabels[i][1];
    Logger.log(label_name + " (" + label_days + " days)");
    
    var label = GmailApp.getUserLabelByName(label_name);
    var threads = label.getThreads();
    for (var j = 0; j < threads.length; j++) {
      var sub = threads[j].getFirstMessageSubject();
      var date = threads[j].getLastMessageDate();
      Logger.log(sub + ": " + date);

      var elapsed = (current - date) / onedaymilli;
      if (elapsed <= label_days) {
        Logger.log(elapsed + " days passed <= " + label_days + " days, safe!");
        continue;
      }
      Logger.log("will be deleted");
      threads[j].moveToTrash();
    }
  }
}