// The name of the Gmail Label that is to be autopurged?
var GMAIL_LABEL = 'Newsletters';

// Purge messages automatically after how many days?
var PURGE_AFTER = '10';

function purgeGmail() {
  var age = new Date();
  age.setDate(age.getDate() - PURGE_AFTER);

  var purge = Utilities.formatDate(age, Session.getTimeZone(), 'yyyy-MM-dd');
  var search = 'label:' + GMAIL_LABEL + ' before:' + purge;

  // This will create a simple Gmail search
  // query like label:Newsletters before:10/12/2012

  try {
    // We are processing 100 messages in a batch to prevent script errors.
    // Else it may throw Exceed Maximum Execution Time exception in Apps Script

    var threads = GmailApp.search(search, 0, 100);

    // For large batches, create another time-based trigger that will
    // activate the auto-purge process after 'n' minutes.

    if (threads.length == 100) {
      ScriptApp.newTrigger('purgeGmail')
        .timeBased()
        .at(new Date(new Date().getTime() + 1000 * 60 * 10))
        .create();
    }

    // An email thread may have multiple messages and the timestamp of
    // individual messages can be different.

    for (var i = 0; i < threads.length; i++) {
      var messages = GmailApp.getMessagesForThread(threads[i]);
      for (var j = 0; j < messages.length; j++) {
        var email = messages[j];
        if (email.getDate() < age) {
          email.moveToTrash();
        }
      }
    }

    // If the script fails for some reason or catches an exception,
    // it will simply defer auto-purge until the next day.
  } catch (e) {}
}