code: function formatTimeString_(time, endTime) {
    var ret = '';
    if (time) {
      ret += (time.getTime() - time.getTimezoneOffset() * 60000);
    }
    if (endTime) {
      ret += ', ' + (endTime.getTime() - endTime.getTimezoneOffset() * 60000);
    }
    return ret;
  }
docs: get REST format for 2 time
  @param {Date} time
  @param {Date} endTime