diff --git a/content/media/webm/nsWebMBufferedParser.cpp b/content/media/webm/nsWebMBufferedParser.cpp
index cbc8e2be4f4..834e46c765a 100644
--- a/content/media/webm/nsWebMBufferedParser.cpp
+++ b/content/media/webm/nsWebMBufferedParser.cpp
@@ -209,10 +209,8 @@ void nsWebMBufferedParser::Append(const unsigned char* aBuffer, PRUint32 aLength
   mCurrentOffset += aLength;
 }
 
-void nsWebMBufferedState::CalculateBufferedForRange(nsTimeRanges* aBuffered,
-                                                    PRInt64 aStartOffset, PRInt64 aEndOffset,
-                                                    PRUint64 aTimecodeScale,
-                                                    PRInt64 aStartTimeOffsetNS)
+bool nsWebMBufferedState::CalculateBufferedForRange(PRInt64 aStartOffset, PRInt64 aEndOffset,
+                                                    PRUint64* aStartTime, PRUint64* aEndTime)
 {
   ReentrantMonitorAutoEnter mon(mReentrantMonitor);
 
@@ -220,7 +218,7 @@ void nsWebMBufferedState::CalculateBufferedForRange(nsTimeRanges* aBuffered,
   PRUint32 start;
   mTimeMapping.GreatestIndexLtEq(aStartOffset, start);
   if (start == mTimeMapping.Length()) {
-    return;
+    return false;
   }
 
   // Find the first nsWebMTimeDataOffset at or before aEndOffset.
@@ -233,7 +231,7 @@ void nsWebMBufferedState::CalculateBufferedForRange(nsTimeRanges* aBuffered,
 
   // Range is empty.
   if (end <= start) {
-    return;
+    return false;
   }
 
   NS_ASSERTION(mTimeMapping[start].mOffset >= aStartOffset &&
@@ -252,9 +250,9 @@ void nsWebMBufferedState::CalculateBufferedForRange(nsTimeRanges* aBuffered,
   // from the ranges' start and end timestamps, so that those timestamps are
   // normalized in the range [0,duration].
 
-  double startTime = (mTimeMapping[start].mTimecode * aTimecodeScale - aStartTimeOffsetNS) / NS_PER_S;
-  double endTime = (mTimeMapping[end].mTimecode * aTimecodeScale - aStartTimeOffsetNS) / NS_PER_S;
-  aBuffered->Add(startTime, endTime);
+  *aStartTime = mTimeMapping[start].mTimecode;
+  *aEndTime = mTimeMapping[end].mTimecode;
+  return true;
 }
 
 void nsWebMBufferedState::NotifyDataArrived(const char* aBuffer, PRUint32 aLength, PRInt64 aOffset)
diff --git a/content/media/webm/nsWebMBufferedParser.h b/content/media/webm/nsWebMBufferedParser.h
index f25feb5dd48..51b27d4a7ee 100644
--- a/content/media/webm/nsWebMBufferedParser.h
+++ b/content/media/webm/nsWebMBufferedParser.h
@@ -225,10 +225,8 @@ public:
   }
 
   void NotifyDataArrived(const char* aBuffer, PRUint32 aLength, PRInt64 aOffset);
-  void CalculateBufferedForRange(nsTimeRanges* aBuffered,
-                                 PRInt64 aStartOffset, PRInt64 aEndOffset,
-                                 PRUint64 aTimecodeScale,
-                                 PRInt64 aStartTimeOffsetNS);
+  bool CalculateBufferedForRange(PRInt64 aStartOffset, PRInt64 aEndOffset,
+                                 PRUint64* aStartTime, PRUint64* aEndTime);
 
 private:
   // Synchronizes access to the mTimeMapping array.
diff --git a/content/media/webm/nsWebMReader.cpp b/content/media/webm/nsWebMReader.cpp
index e3b3b03cf49..7f56d2337cc 100644
--- a/content/media/webm/nsWebMReader.cpp
+++ b/content/media/webm/nsWebMReader.cpp
@@ -791,24 +791,45 @@ nsresult nsWebMReader::GetBuffered(nsTimeRanges* aBuffered, PRInt64 aStartTime)
   }
 
   // Special case completely cached files.  This also handles local files.
-  if (resource->IsDataCachedToEndOfResource(0)) {
+  bool isFullyCached = resource->IsDataCachedToEndOfResource(0);
+  if (isFullyCached) {
     uint64_t duration = 0;
     if (nestegg_duration(mContext, &duration) == 0) {
       aBuffered->Add(0, duration / NS_PER_S);
     }
-  } else {
+  }
+
+  PRUint32 bufferedLength = 0;
+  aBuffered->GetLength(&bufferedLength);
+
+  // Either we the file is not fully cached, or we couldn't find a duration in
+  // the WebM bitstream.
+  if (!isFullyCached || !bufferedLength) {
     MediaResource* resource = mDecoder->GetResource();
     nsTArray<MediaByteRange> ranges;
     nsresult res = resource->GetCachedRanges(ranges);
     NS_ENSURE_SUCCESS(res, res);
 
-    PRInt64 startTimeOffsetNS = aStartTime * NS_PER_USEC;
     for (PRUint32 index = 0; index < ranges.Length(); index++) {
-      mBufferedState->CalculateBufferedForRange(aBuffered,
-                                                ranges[index].mStart,
-                                                ranges[index].mEnd,
-                                                timecodeScale,
-                                                startTimeOffsetNS);
+      PRUint64 start, end;
+      bool rv = mBufferedState->CalculateBufferedForRange(ranges[index].mStart,
+                                                          ranges[index].mEnd,
+                                                          &start, &end);
+      if (rv) {
+        double startTime = start * timecodeScale / NS_PER_S - aStartTime;
+        double endTime = end * timecodeScale / NS_PER_S - aStartTime;
+
+        // If this range extends to the end of the file, the true end time
+        // is the file's duration.
+        if (resource->IsDataCachedToEndOfResource(ranges[index].mStart)) {
+          uint64_t duration = 0;
+          if (nestegg_duration(mContext, &duration) == 0) {
+            endTime = duration / NS_PER_S;
+          }
+        }
+
+        aBuffered->Add(startTime, endTime);
+      }
     }
   }