diff --git a/src/main/java/io/numaproj/numaflow/accumulator/AccumulatorActor.java b/src/main/java/io/numaproj/numaflow/accumulator/AccumulatorActor.java
index 3dbf611d..e0e38539 100644
--- a/src/main/java/io/numaproj/numaflow/accumulator/AccumulatorActor.java
+++ b/src/main/java/io/numaproj/numaflow/accumulator/AccumulatorActor.java
@@ -50,6 +50,8 @@ private void sendEOF(String EOF) {
                 .newBuilder()
                 .setWindow(AccumulatorOuterClass.KeyedWindow
                         .newBuilder()
+                        .setStart(this.keyedWindow.getStart())
+                        .setEnd(this.keyedWindow.getEnd())
                         .addAllKeys(this.keyedWindow.getKeysList()))
                 .setEOF(true)
                 .build();
diff --git a/src/main/java/io/numaproj/numaflow/accumulator/OutputStreamObserverImpl.java b/src/main/java/io/numaproj/numaflow/accumulator/OutputStreamObserverImpl.java
index 24811de6..d1ea040e 100644
--- a/src/main/java/io/numaproj/numaflow/accumulator/OutputStreamObserverImpl.java
+++ b/src/main/java/io/numaproj/numaflow/accumulator/OutputStreamObserverImpl.java
@@ -15,7 +15,7 @@
 class OutputStreamObserverImpl implements OutputStreamObserver {
     private final ActorRef outputActor;
     private final AccumulatorOuterClass.KeyedWindow keyedWindow;
-    private final Instant latestWatermark = Instant.ofEpochMilli(-1);
+    private Instant latestWatermark = Instant.ofEpochMilli(-1);
 
     public OutputStreamObserverImpl(
             ActorRef outputActor,
@@ -26,14 +26,17 @@ public OutputStreamObserverImpl(
 
     private AccumulatorOuterClass.AccumulatorResponse buildResponse(Message message) {
         AccumulatorOuterClass.AccumulatorResponse.Builder responseBuilder = AccumulatorOuterClass.AccumulatorResponse.newBuilder();
+        if (message.getWatermark().isAfter(latestWatermark)) {
+            this.latestWatermark = message.getWatermark();
+        }
         // set the window using the actor metadata.
         responseBuilder.setWindow(AccumulatorOuterClass.KeyedWindow.newBuilder()
                 .setStart(Timestamp.newBuilder()
                         .setSeconds(0)
-                        .setNanos(0))
+                        .setNanos(0).build())
                 .setEnd(Timestamp.newBuilder()
                         .setSeconds(this.latestWatermark.getEpochSecond())
-                        .setNanos(this.latestWatermark.getNano()))
+                        .setNanos(this.latestWatermark.getNano()).build())
                 .setSlot("slot-0")
                 .addAllKeys(this.keyedWindow.getKeysList())
                 .build());
@@ -47,11 +50,11 @@ private AccumulatorOuterClass.AccumulatorResponse buildResponse(Message message)
                         .setEventTime(Timestamp
                                 .newBuilder()
                                 .setSeconds(message.getEventTime().getEpochSecond())
-                                .setNanos(message.getEventTime().getNano()))
+                                .setNanos(message.getEventTime().getNano()).build())
                         .setWatermark(Timestamp
                                 .newBuilder()
                                 .setSeconds(message.getWatermark().getEpochSecond())
-                                .setNanos(message.getWatermark().getNano()))
+                                .setNanos(message.getWatermark().getNano()).build())
                         .putAllHeaders(message.getHeaders())
                         .addAllKeys(message.getKeys()
                                 == null ? new ArrayList<>() : Arrays.asList(message.getKeys()))