diff --git a/gfx/layers/AnimationHelper.cpp b/gfx/layers/AnimationHelper.cpp
index 12c3529f87b8..554a8b427503 100644
--- a/gfx/layers/AnimationHelper.cpp
+++ b/gfx/layers/AnimationHelper.cpp
@@ -166,8 +166,7 @@ static AnimationHelper::SampleResult SampleAnimationForProperty(
   // Process in order, since later animations override earlier ones.
   for (PropertyAnimation& animation : aPropertyAnimations) {
     MOZ_ASSERT(
-        (!animation.mOriginTime.IsNull() &&
-         animation.mStartTime.type() == MaybeTimeDuration::TTimeDuration) ||
+        (!animation.mOriginTime.IsNull() && animation.mStartTime.isSome()) ||
             animation.mIsNotPlaying,
         "If we are playing, we should have an origin time and a start time");
 
@@ -192,7 +191,7 @@ static AnimationHelper::SampleResult SampleAnimationForProperty(
       // underflow in the middle of the calulation.
       const TimeStamp readyTime =
           animation.mOriginTime +
-          (animation.mStartTime.get_TimeDuration() +
+          (animation.mStartTime.ref() +
            animation.mHoldTime.MultDouble(1.0 / animation.mPlaybackRate));
       hasFutureReadyTime =
           !readyTime.IsNull() && readyTime > aPreviousFrameTime;
@@ -216,11 +215,9 @@ static AnimationHelper::SampleResult SampleAnimationForProperty(
     // If the animation is not currently playing, e.g. paused or
     // finished, then use the hold time to stay at the same position.
     TimeDuration elapsedDuration =
-        animation.mIsNotPlaying ||
-                animation.mStartTime.type() != MaybeTimeDuration::TTimeDuration
+        animation.mIsNotPlaying || animation.mStartTime.isNothing()
             ? animation.mHoldTime
-            : (timeStamp - animation.mOriginTime -
-               animation.mStartTime.get_TimeDuration())
+            : (timeStamp - animation.mOriginTime - animation.mStartTime.ref())
                   .MultDouble(animation.mPlaybackRate);
 
     ComputedTiming computedTiming = dom::AnimationEffect::GetComputedTimingAt(
diff --git a/gfx/layers/AnimationHelper.h b/gfx/layers/AnimationHelper.h
index ec374b1ddf37..29c614ebb3c9 100644
--- a/gfx/layers/AnimationHelper.h
+++ b/gfx/layers/AnimationHelper.h
@@ -52,7 +52,7 @@ struct PropertyAnimation {
   dom::Nullable<double> mPortionInSegmentOnLastCompose;
 
   TimeStamp mOriginTime;
-  MaybeTimeDuration mStartTime;
+  Maybe<TimeDuration> mStartTime;
   TimeDuration mHoldTime;
   float mPlaybackRate;
   dom::IterationCompositeOperation mIterationComposite;
diff --git a/gfx/layers/AnimationInfo.cpp b/gfx/layers/AnimationInfo.cpp
index 877983aec574..ec50e75ab17b 100644
--- a/gfx/layers/AnimationInfo.cpp
+++ b/gfx/layers/AnimationInfo.cpp
@@ -88,23 +88,21 @@ bool AnimationInfo::StartPendingAnimations(const TimeStamp& aReadyTime) {
 
     // If the animation is doing an async update of its playback rate, then we
     // want to match whatever its current time would be at *aReadyTime*.
-    if (!std::isnan(anim.previousPlaybackRate()) &&
-        anim.startTime().type() == MaybeTimeDuration::TTimeDuration &&
+    if (!std::isnan(anim.previousPlaybackRate()) && anim.startTime().isSome() &&
         !anim.originTime().IsNull() && !anim.isNotPlaying()) {
       TimeDuration readyTime = aReadyTime - anim.originTime();
       anim.holdTime() = dom::Animation::CurrentTimeFromTimelineTime(
-          readyTime, anim.startTime().get_TimeDuration(),
-          anim.previousPlaybackRate());
+          readyTime, anim.startTime().ref(), anim.previousPlaybackRate());
       // Make start time null so that we know to update it below.
-      anim.startTime() = null_t();
+      anim.startTime() = Nothing();
     }
 
     // If the animation is play-pending, resolve the start time.
-    if (anim.startTime().type() == MaybeTimeDuration::Tnull_t &&
-        !anim.originTime().IsNull() && !anim.isNotPlaying()) {
+    if (anim.startTime().isNothing() && !anim.originTime().IsNull() &&
+        !anim.isNotPlaying()) {
       TimeDuration readyTime = aReadyTime - anim.originTime();
-      anim.startTime() = dom::Animation::StartTimeFromTimelineTime(
-          readyTime, anim.holdTime(), anim.playbackRate());
+      anim.startTime() = Some(dom::Animation::StartTimeFromTimelineTime(
+          readyTime, anim.holdTime(), anim.playbackRate()));
       updated = true;
     }
   }
diff --git a/gfx/layers/ipc/LayersMessages.ipdlh b/gfx/layers/ipc/LayersMessages.ipdlh
index dc445e2eb929..0afe4bc492f5 100644
--- a/gfx/layers/ipc/LayersMessages.ipdlh
+++ b/gfx/layers/ipc/LayersMessages.ipdlh
@@ -162,11 +162,6 @@ union TransformFunction {
   TransformMatrix;
 };
 
-union MaybeTimeDuration {
-  null_t;
-  TimeDuration;
-};
-
 union Animatable {
   null_t;
   float;
@@ -214,7 +209,7 @@ struct Animation {
   TimeStamp originTime;
   // The start time is relative to the originTime. This allows us to represent
   // start times in the distant past that cannot be expressed using a TimeStamp.
-  MaybeTimeDuration startTime;
+  TimeDuration? startTime;
   TimeDuration delay;
   TimeDuration endDelay;
   // The value of the animation's current time at the moment it was sent to the
diff --git a/layout/painting/nsDisplayList.cpp b/layout/painting/nsDisplayList.cpp
index 762833b1cb05..1255ba083147 100644
--- a/layout/painting/nsDisplayList.cpp
+++ b/layout/painting/nsDisplayList.cpp
@@ -570,9 +570,9 @@ static void AddAnimationForProperty(nsIFrame* aFrame,
 
   Nullable<TimeDuration> startTime = aAnimation->GetCurrentOrPendingStartTime();
   if (startTime.IsNull()) {
-    animation->startTime() = null_t();
+    animation->startTime() = Nothing();
   } else {
-    animation->startTime() = startTime.Value();
+    animation->startTime() = Some(startTime.Value());
   }
 
   animation->holdTime() = aAnimation->GetCurrentTimeAsDuration().Value();