App Script to move old Gmail threads to Trash

GmailIndef.gs

function GmailIndef() { 
  var INDEF_LABEL = 'Indef';
  
  var RETENTION_DAYS = 356;
  var RETENTION_DATE = new Date();
  RETENTION_DATE.setDate(RETENTION_DATE.getDate() - RETENTION_DAYS);
  
  var threads = GmailApp.search('in:anywhere'
                  + ' -(label:' + INDEF_LABEL + ')'
                  + ' before:' + RETENTION_DATE.getFullYear()
                          + '-' + (RETENTION_DATE.getMonth() + 1)
                          + '-' + RETENTION_DATE.getDate());
  
  threads.forEach(function(thread) {
    GmailApp.moveThreadToTrash(thread);
  });
}