diff --git a/core/src/main/java/com/google/adk/sessions/InMemorySessionService.java b/core/src/main/java/com/google/adk/sessions/InMemorySessionService.java
index b2a584b11..da338ed57 100644
--- a/core/src/main/java/com/google/adk/sessions/InMemorySessionService.java
+++ b/core/src/main/java/com/google/adk/sessions/InMemorySessionService.java
@@ -163,8 +163,8 @@ public Maybe<Session> getSession(
   }
 
   // Helper to get event timestamp as epoch seconds
-  private long getEventTimestampEpochSeconds(Event event) {
-    return event.timestamp() / 1000L;
+  private double getEventTimestampEpochSeconds(Event event) {
+    return event.timestamp() / 1000.0;
   }
 
   @Override
diff --git a/core/src/test/java/com/google/adk/sessions/InMemorySessionServiceTest.java b/core/src/test/java/com/google/adk/sessions/InMemorySessionServiceTest.java
index 41e156ffd..0d9235b1b 100644
--- a/core/src/test/java/com/google/adk/sessions/InMemorySessionServiceTest.java
+++ b/core/src/test/java/com/google/adk/sessions/InMemorySessionServiceTest.java
@@ -20,6 +20,7 @@
 import com.google.adk.events.Event;
 import com.google.adk.events.EventActions;
 import io.reactivex.rxjava3.core.Single;
+import java.time.Instant;
 import java.util.HashMap;
 import java.util.Optional;
 import java.util.concurrent.ConcurrentHashMap;
@@ -214,6 +215,24 @@ public void appendEvent_removesState() {
     assertThat(retrievedSessionRemove.state()).doesNotContainKey("temp:tempKey");
   }
 
+  @Test
+  public void appendEvent_updatesSessionTimestampWithFractionalSeconds() {
+    InMemorySessionService sessionService = new InMemorySessionService();
+    Session session =
+        sessionService.createSession("app", "user", new HashMap<>(), "session1").blockingGet();
+
+    // Add an event with a timestamp that contains a fractional second
+    Event eventAdd = Event.builder().timestamp(5500).build();
+    var unused = sessionService.appendEvent(session, eventAdd).blockingGet();
+
+    // Verify the last modified timestamp contains a fractional second
+    Session retrievedSession =
+        sessionService
+            .getSession(session.appName(), session.userId(), session.id(), Optional.empty())
+            .blockingGet();
+    assertThat(retrievedSession.lastUpdateTime()).isEqualTo(Instant.ofEpochSecond(5, 500000000L));
+  }
+
   @Test
   public void sequentialAgents_shareTempState() {
     InMemorySessionService sessionService = new InMemorySessionService();