getProgramStartTime_() {
    if (this.mediaElement_.getStartDate) {
      const startDate = this.mediaElement_.getStartDate();
      const startTime = startDate.getTime();
      if (!isNaN(startTime)) {
        return startTime / 1000.0;
      }
    }
    return null;
  }
@return {?number} program start time in seconds.
@private