diff --git a/src/test/java/net/javahippie/fitpub/controller/ActivityPubControllerTest.java b/src/test/java/net/javahippie/fitpub/controller/ActivityPubControllerTest.java
new file mode 100644
index 0000000..eb52f67
--- /dev/null
+++ b/src/test/java/net/javahippie/fitpub/controller/ActivityPubControllerTest.java
@@ -0,0 +1,114 @@
+package net.javahippie.fitpub.controller;
+
+import com.fasterxml.jackson.databind.ObjectMapper;
+import net.javahippie.fitpub.model.entity.Activity;
+import net.javahippie.fitpub.model.entity.User;
+import net.javahippie.fitpub.repository.ActivityRepository;
+import net.javahippie.fitpub.repository.FollowRepository;
+import net.javahippie.fitpub.repository.UserRepository;
+import net.javahippie.fitpub.security.HttpSignatureValidator;
+import net.javahippie.fitpub.service.ActivityImageService;
+import net.javahippie.fitpub.service.FederationService;
+import net.javahippie.fitpub.service.InboxProcessor;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.InjectMocks;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+import org.springframework.http.ResponseEntity;
+import org.springframework.test.util.ReflectionTestUtils;
+
+import java.io.File;
+import java.math.BigDecimal;
+import java.time.LocalDateTime;
+import java.time.ZoneOffset;
+import java.util.Map;
+import java.util.Optional;
+import java.util.UUID;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.Mockito.when;
+
+@ExtendWith(MockitoExtension.class)
+@DisplayName("ActivityPubController Tests")
+class ActivityPubControllerTest {
+
+    @Mock
+    private UserRepository userRepository;
+
+    @Mock
+    private ActivityRepository activityRepository;
+
+    @Mock
+    private ActivityImageService activityImageService;
+
+    @Mock
+    private InboxProcessor inboxProcessor;
+
+    @Mock
+    private FollowRepository followRepository;
+
+    @Mock
+    private HttpSignatureValidator signatureValidator;
+
+    @Mock
+    private FederationService federationService;
+
+    @Mock
+    private ObjectMapper objectMapper;
+
+    @InjectMocks
+    private ActivityPubController controller;
+
+    private UUID activityId;
+    private UUID userId;
+    private Activity activity;
+    private User user;
+    private LocalDateTime createdAt;
+
+    @BeforeEach
+    void setUp() {
+        activityId = UUID.randomUUID();
+        userId = UUID.randomUUID();
+        createdAt = LocalDateTime.of(2026, 5, 2, 9, 24, 50, 921_241_000);
+
+        ReflectionTestUtils.setField(controller, "baseUrl", "https://fitpub.example");
+
+        activity = Activity.builder()
+            .id(activityId)
+            .userId(userId)
+            .activityType(Activity.ActivityType.RUN)
+            .title("Lunch Run")
+            .description("Sunny run")
+            .visibility(Activity.Visibility.PUBLIC)
+            .totalDistance(BigDecimal.valueOf(5000))
+            .totalDurationSeconds(1800L)
+            .createdAt(createdAt)
+            .build();
+
+        user = User.builder()
+            .id(userId)
+            .username("JaneDoe")
+            .email("janedoe@example.com")
+            .publicKey("public-key")
+            .privateKey("private-key")
+            .build();
+    }
+
+    @Test
+    @DisplayName("Should serialize activity published timestamp with timezone")
+    void getActivity_ShouldSerializePublishedTimestampWithTimezone() {
+        when(activityRepository.findById(activityId)).thenReturn(Optional.of(activity));
+        when(userRepository.findById(userId)).thenReturn(Optional.of(user));
+        when(activityImageService.getActivityImageFile(activityId)).thenReturn(new File("/definitely/nonexistent-fitpub-test-image"));
+
+        ResponseEntity<Map<String, Object>> response = controller.getActivity(activityId);
+
+        assertThat(response.getStatusCode().is2xxSuccessful()).isTrue();
+        assertThat(response.getBody()).isNotNull();
+        assertThat(response.getBody().get("published"))
+            .isEqualTo(createdAt.atOffset(ZoneOffset.UTC).toInstant().toString());
+    }
+}
diff --git a/src/test/java/net/javahippie/fitpub/service/ActivityPostProcessingServiceTest.java b/src/test/java/net/javahippie/fitpub/service/ActivityPostProcessingServiceTest.java
index 08ef492..b2e6c84 100644
--- a/src/test/java/net/javahippie/fitpub/service/ActivityPostProcessingServiceTest.java
+++ b/src/test/java/net/javahippie/fitpub/service/ActivityPostProcessingServiceTest.java
@@ -4,6 +4,7 @@ import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.DisplayName;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.ArgumentCaptor;
 import org.mockito.InjectMocks;
 import org.mockito.Mock;
 import org.mockito.junit.jupiter.MockitoExtension;
@@ -15,9 +16,11 @@ import org.springframework.test.util.ReflectionTestUtils;
 
 import java.math.BigDecimal;
 import java.time.LocalDateTime;
+import java.time.ZoneOffset;
 import java.util.Optional;
 import java.util.UUID;
 
+import static org.assertj.core.api.Assertions.assertThat;
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.Mockito.*;
 
@@ -56,11 +59,13 @@ class ActivityPostProcessingServiceTest {
     private UUID userId;
     private Activity testActivity;
     private User testUser;
+    private LocalDateTime createdAt;
 
     @BeforeEach
     void setUp() {
         activityId = UUID.randomUUID();
         userId = UUID.randomUUID();
+        createdAt = LocalDateTime.of(2026, 5, 2, 9, 24, 50, 921_241_000);
 
         // Set baseUrl via reflection (since it's @Value injected)
         ReflectionTestUtils.setField(service, "baseUrl", "https://test.example");
@@ -76,8 +81,8 @@ class ActivityPostProcessingServiceTest {
             .totalDistance(BigDecimal.valueOf(5000))
             .totalDurationSeconds(1800L)
             .elevationGain(BigDecimal.valueOf(100))
-            .startedAt(LocalDateTime.now())
-            .createdAt(LocalDateTime.now())
+            .startedAt(createdAt.minusMinutes(30))
+            .createdAt(createdAt)
             .build();
 
         // Create test user
@@ -232,6 +237,24 @@ class ActivityPostProcessingServiceTest {
         verify(federationService).sendCreateActivity(anyString(), any(), eq(testUser), eq(false));
     }
 
+    @Test
+    @DisplayName("Should serialize federation note published timestamp with timezone")
+    void testPublishToFederationAsync_PublishedTimestampIncludesTimezone() {
+        when(activityRepository.findById(activityId)).thenReturn(Optional.of(testActivity));
+        when(userRepository.findById(userId)).thenReturn(Optional.of(testUser));
+        when(activityImageService.generateActivityImage(testActivity)).thenReturn(null);
+        doNothing().when(federationService).sendCreateActivity(anyString(), any(), any(), anyBoolean());
+
+        @SuppressWarnings("unchecked")
+        ArgumentCaptor<java.util.Map<String, Object>> noteCaptor = ArgumentCaptor.forClass(java.util.Map.class);
+
+        service.publishToFederationAsync(activityId, userId);
+
+        verify(federationService).sendCreateActivity(anyString(), noteCaptor.capture(), eq(testUser), eq(true));
+        assertThat(noteCaptor.getValue().get("published"))
+            .isEqualTo(createdAt.atOffset(ZoneOffset.UTC).toInstant().toString());
+    }
+
     @Test
     @DisplayName("Should skip federation for PRIVATE activity")
     void testPublishToFederationAsync_PrivateActivity() {