// The name of the Gmail Label to expire
var GMAIL_LABEL = 'expires';

// Archive messages automatically after N days
var EXPIRE_AFTER = '10';

/*
 * STOP EDITING BELOW HERE UNLESS 
 * YOU KNOW WHAT YOU'RE DOING
 *
 */

function Install() {
  ScriptApp.newTrigger('purge')
    .timeBased()
    .after(1000 * 60 * 5)
    .create()
  ;
  
  ScriptApp.newTrigger('purge')
    .timeBased()
    .everyDays(1)
    .create()
  ;
}

function Uninstall() {  
  var 
    triggers = ScriptApp.getProjectTriggers()
    ,i = 0
    ,trigger
  ;
  
  while (trigger = triggers[i++]) {
    ScriptApp.deleteTrigger(trigger);
  }
}

function purge() {
  var 
    expiry = new Date
    ,i = 0
    ,threads
    ,thread
  ;
  
  expiry.setDate(expiry.getDate() - EXPIRE_AFTER);    
 
  threads = GmailApp.search('in:inbox label:' + GMAIL_LABEL + ' before:' + Utilities.formatDate(expiry, Session.getScriptTimeZone(), 'yyyy-MM-dd'), 0, 100);
  
  if (threads.length == 100) {
    ScriptApp.newTrigger('purge')
      .timeBased()
      .after(1000 * 60 * 5)
      .create()
    ;
  }
  
  while (thread = threads[i++]) {
    if (thread.getLastMessageDate() < expiry) {
      thread.moveToArchive();
    }
  }
}