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..d9bb047a3 100644
--- a/core/src/main/java/com/google/adk/sessions/InMemorySessionService.java
+++ b/core/src/main/java/com/google/adk/sessions/InMemorySessionService.java
@@ -154,19 +154,13 @@ public Maybe<Session> getSession(
     if (config.numRecentEvents().isEmpty() && config.afterTimestamp().isPresent()) {
       Instant threshold = config.afterTimestamp().get();
 
-      eventsInCopy.removeIf(
-          event -> getEventTimestampEpochSeconds(event) < threshold.getEpochSecond());
+      eventsInCopy.removeIf(event -> getInstantFromEvent(event).isBefore(threshold));
     }
 
     // Merge state into the potentially filtered copy and return
     return Maybe.just(mergeWithGlobalState(appName, userId, sessionCopy));
   }
 
-  // Helper to get event timestamp as epoch seconds
-  private long getEventTimestampEpochSeconds(Event event) {
-    return event.timestamp() / 1000L;
-  }
-
   @Override
   public Single<ListSessionsResponse> listSessions(String appName, String userId) {
     Objects.requireNonNull(appName, "appName cannot be null");
@@ -294,10 +288,7 @@ public Single<Event> appendEvent(Session session, Event event) {
   /** Converts an event's timestamp to an Instant. Adapt based on actual Event structure. */
   // TODO: have Event.timestamp() return Instant directly
   private Instant getInstantFromEvent(Event event) {
-    double epochSeconds = getEventTimestampEpochSeconds(event);
-    long seconds = (long) epochSeconds;
-    long nanos = (long) ((epochSeconds % 1.0) * 1_000_000_000L);
-    return Instant.ofEpochSecond(seconds, nanos);
+    return Instant.ofEpochMilli(event.timestamp());
   }
 
   /**
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();