diff --git a/layout/style/nsAnimationManager.cpp b/layout/style/nsAnimationManager.cpp
index 1ee6eb266ac..eb1e2fca559 100644
--- a/layout/style/nsAnimationManager.cpp
+++ b/layout/style/nsAnimationManager.cpp
@@ -309,6 +309,30 @@ CSSAnimation::UpdateTiming(SeekFlag aSeekFlag, SyncNotifyFlag aSyncNotifyFlag)
   Animation::UpdateTiming(aSeekFlag, aSyncNotifyFlag);
 }
 
+TimeStamp
+CSSAnimation::ElapsedTimeToTimeStamp(const StickyTimeDuration&
+                                       aElapsedTime) const
+{
+  // Initializes to null. We always return this object to benefit from
+  // return-value-optimization.
+  TimeStamp result;
+
+  // Currently we may dispatch animationstart events before resolving
+  // mStartTime if we have a delay <= 0. This will change in bug 1134163
+  // but until then we should just use the latest refresh driver time as
+  // the event timestamp in that case.
+  if (!mEffect || mStartTime.IsNull()) {
+    nsPresContext* presContext = GetPresContext();
+    if (presContext) {
+      result = presContext->RefreshDriver()->MostRecentRefresh();
+    }
+    return result;
+  }
+
+  result = AnimationTimeToTimeStamp(aElapsedTime + mEffect->Timing().mDelay);
+  return result;
+}
+
 ////////////////////////// nsAnimationManager ////////////////////////////
 
 NS_IMPL_CYCLE_COLLECTION(nsAnimationManager, mEventDispatcher)
diff --git a/layout/style/nsAnimationManager.h b/layout/style/nsAnimationManager.h
index 536ae15f87a..ba29d5fbd45 100644
--- a/layout/style/nsAnimationManager.h
+++ b/layout/style/nsAnimationManager.h
@@ -176,6 +176,10 @@ protected:
            std::max(TimeDuration(), mEffect->Timing().mDelay * -1) :
            TimeDuration();
   }
+  // Converts an AnimationEvent's elapsedTime value to an equivalent TimeStamp
+  // that can be used to sort events by when they occurred.
+  TimeStamp ElapsedTimeToTimeStamp(const StickyTimeDuration& aElapsedTime)
+    const;
 
   nsString mAnimationName;