diff --git a/src/main/java/net/javahippie/fitpub/controller/ActivityPubController.java b/src/main/java/net/javahippie/fitpub/controller/ActivityPubController.java
index 4cf3717..9702429 100644
--- a/src/main/java/net/javahippie/fitpub/controller/ActivityPubController.java
+++ b/src/main/java/net/javahippie/fitpub/controller/ActivityPubController.java
@@ -29,6 +29,7 @@ import java.net.URI;
 import java.net.URISyntaxException;
 import java.security.MessageDigest;
 import java.security.NoSuchAlgorithmException;
+import java.time.ZoneOffset;
 import java.util.*;
 import java.util.regex.Pattern;
 
@@ -436,7 +437,7 @@ public class ActivityPubController {
         noteObject.put("id", activityUri);
         noteObject.put("type", "Note");
         noteObject.put("attributedTo", actorUri);
-        noteObject.put("published", activity.getCreatedAt().toString());
+        noteObject.put("published", activity.getCreatedAt().atOffset(ZoneOffset.UTC).toInstant().toString());
         noteObject.put("content", formatActivityContent(activity));
         noteObject.put("url", activityUri);
 
diff --git a/src/main/java/net/javahippie/fitpub/service/ActivityPostProcessingService.java b/src/main/java/net/javahippie/fitpub/service/ActivityPostProcessingService.java
index 8a582bc..eb0ec99 100644
--- a/src/main/java/net/javahippie/fitpub/service/ActivityPostProcessingService.java
+++ b/src/main/java/net/javahippie/fitpub/service/ActivityPostProcessingService.java
@@ -12,6 +12,7 @@ import org.springframework.beans.factory.annotation.Value;
 import org.springframework.scheduling.annotation.Async;
 import org.springframework.stereotype.Service;
 
+import java.time.ZoneOffset;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
@@ -199,7 +200,7 @@ public class ActivityPostProcessingService {
             noteObject.put("id", activityUri);
             noteObject.put("type", "Note");
             noteObject.put("attributedTo", actorUri);
-            noteObject.put("published", activity.getCreatedAt().toString());
+            noteObject.put("published", activity.getCreatedAt().atOffset(ZoneOffset.UTC).toInstant().toString());
             noteObject.put("content", formatActivityContent(activity));
             noteObject.put("url", baseUrl + "/activities/" + activity.getId());