Gmail AutoArchive.js

// Google Apps Script for use at https://script.google.com/
// Use timed triggers to run the desired functions on a regular schedule (e.g. every 12 hours)

// search is a gmail search string, keepDays is the last N days of threads to keep in Inbox
function gmailAutoArchive(search, keepDays) {
  var maxDate = new Date();
  maxDate.setDate(maxDate.getDate()-keepDays);
  
  var threads = GmailApp.search(search);
  
  // archive all the threads where the last message is before maxDate
  for (var i = 0; i < threads.length; i++) {
    if (threads[i].getLastMessageDate()<maxDate)
    {
      threads[i].markRead();
      threads[i].moveToArchive();
      Logger.log("Archived: %s", threads[i].getFirstMessageSubject());
    }
  }
}

function gmailAutoArchiveRead() {
   // limit archiving to read threads, where the last message is more than 14 days ago
  gmailAutoArchive('in:inbox is:read', 14);
}

function gmailAutoArchiveNotifications() {
  // limit archiving to threads with the notifications label, where the last message is more than 7 days ago
  gmailAutoArchive('label:notifications', 7);
}