diff --git a/dom/base/File.cpp b/dom/base/File.cpp
index c1b35c50997..1cbe0122dd0 100644
--- a/dom/base/File.cpp
+++ b/dom/base/File.cpp
@@ -616,6 +616,10 @@ File::Constructor(
   }
   MOZ_ASSERT(impl->IsFile());
 
+  if (aBag.mLastModified.WasPassed()) {
+    impl->SetLastModified(aBag.mLastModified.Value());
+  }
+
   nsRefPtr<File> file = new File(aGlobal.GetAsSupports(), impl);
   return file.forget();
 }
@@ -792,6 +796,12 @@ FileImplBase::GetLastModified(ErrorResult& aRv)
   return mLastModificationDate / PR_USEC_PER_MSEC;
 }
 
+void
+FileImplBase::SetLastModified(int64_t aLastModified)
+{
+  mLastModificationDate = aLastModified * PR_USEC_PER_MSEC;
+}
+
 int64_t
 FileImplBase::GetFileId()
 {
@@ -1015,6 +1025,12 @@ FileImplFile::GetLastModified(ErrorResult& aRv)
   return mLastModificationDate;
 }
 
+void
+FileImplFile::SetLastModified(int64_t aLastModified)
+{
+  MOZ_CRASH("SetLastModified of a real file is not allowed!");
+}
+
 const uint32_t sFileStreamFlags =
   nsIFileInputStream::CLOSE_ON_EOF |
   nsIFileInputStream::REOPEN_ON_REWIND |
diff --git a/dom/base/File.h b/dom/base/File.h
index 414388b227e..3702fefebd2 100644
--- a/dom/base/File.h
+++ b/dom/base/File.h
@@ -236,6 +236,8 @@ public:
 
   virtual int64_t GetLastModified(ErrorResult& aRv) = 0;
 
+  virtual void SetLastModified(int64_t aLastModified) = 0;
+
   virtual void GetMozFullPath(nsAString& aName, ErrorResult& aRv) = 0;
 
   virtual void GetMozFullPathInternal(nsAString& aFileName, ErrorResult& aRv) = 0;
@@ -362,6 +364,8 @@ public:
 
   virtual int64_t GetLastModified(ErrorResult& aRv) MOZ_OVERRIDE;
 
+  virtual void SetLastModified(int64_t aLastModified) MOZ_OVERRIDE;
+
   virtual void GetMozFullPath(nsAString& aName, ErrorResult& aRv) MOZ_OVERRIDE;
 
   virtual void GetMozFullPathInternal(nsAString& aFileName,
@@ -738,6 +742,7 @@ public:
   virtual uint64_t GetSize(ErrorResult& aRv) MOZ_OVERRIDE;
   virtual void GetType(nsAString& aType) MOZ_OVERRIDE;
   virtual int64_t GetLastModified(ErrorResult& aRv) MOZ_OVERRIDE;
+  virtual void SetLastModified(int64_t aLastModified) MOZ_OVERRIDE;
   virtual void GetMozFullPathInternal(nsAString& aFullPath,
                                       ErrorResult& aRv) MOZ_OVERRIDE;
   virtual nsresult GetInternalStream(nsIInputStream**) MOZ_OVERRIDE;