calculateMSD_() {
    if (!this.msdSent_ &&
        this.playbackPlayingTime_ &&
        this.playbackPlayTime_) {
      const startTime = this.startTimeOfLoad_ || this.playbackPlayTime_;
      return this.playbackPlayingTime_ - startTime;
    }
    return undefined;
  }
Calculate measured start delay

@return {number|undefined}
@private