--- a/dom/media/MediaFormatReader.cpp
+++ b/dom/media/MediaFormatReader.cpp
@@ -1960,37 +1960,23 @@ MediaFormatReader::SetSeekTarget(const S
 {
   MOZ_ASSERT(OnTaskQueue());
 
   SeekTarget target = aTarget;
 
   // Transform the seek target time to the demuxer timeline.
   if (!ForceZeroStartTime()) {
     target.SetTime(aTarget.GetTime() - TimeUnit::FromMicroseconds(StartTime())
-                   + DemuxStartTime());
+                   + mInfo.mStartTime);
   }
 
   mOriginalSeekTarget = target;
   mFallbackSeekTime = mPendingSeekTime = Some(target.GetTime());
 }
 
-TimeUnit
-MediaFormatReader::DemuxStartTime()
-{
-  MOZ_ASSERT(OnTaskQueue());
-  MOZ_ASSERT(!ForceZeroStartTime());
-  MOZ_ASSERT(HasAudio() || HasVideo());
-
-  const TimeUnit startTime =
-    std::min(mAudio.mFirstDemuxedSampleTime.refOr(TimeUnit::FromInfinity()),
-             mVideo.mFirstDemuxedSampleTime.refOr(TimeUnit::FromInfinity()));
-
-  return startTime.IsInfinite() ? TimeUnit::FromMicroseconds(0) : startTime;
-}
-
 void
 MediaFormatReader::ScheduleSeek()
 {
   if (mSeekScheduled) {
     return;
   }
   mSeekScheduled = true;
   OwnerThread()->Dispatch(NewRunnableMethod(this, &MediaFormatReader::AttemptSeek));

--- a/dom/media/MediaFormatReader.h
+++ b/dom/media/MediaFormatReader.h
@@ -548,17 +548,16 @@ private:
   // Set to true if any of our track buffers may be blocking.
   bool mTrackDemuxersMayBlock;
 
   // Set the demuxed-only flag.
   Atomic<bool> mDemuxOnly;
 
   // Seeking objects.
   void SetSeekTarget(const SeekTarget& aTarget);
-  media::TimeUnit DemuxStartTime();
   bool IsSeeking() const { return mPendingSeekTime.isSome(); }
   bool IsVideoSeeking() const
   {
     return IsSeeking() && mOriginalSeekTarget.IsVideoOnly();
   }
   void ScheduleSeek();
   void AttemptSeek();
   void OnSeekFailed(TrackType aTrack, const MediaResult& aError);