diff --git a/VIBE/CHAT_LOG.md b/VIBE/CHAT_LOG.md
index 42564b4..ca7d952 100644
--- a/VIBE/CHAT_LOG.md
+++ b/VIBE/CHAT_LOG.md
@@ -1,4 +1,4 @@
-# Chat Log
+
 
 ### User
 The release server fails because its curl does not support `--fail-with-body`.
@@ -1561,3 +1561,11 @@ In the toolbar switch the search and tag filter's location
 
 ### Assistant outcome
 Swapped the toolbar positions of the Search and Tag filter controls while preserving their existing IDs and behavior. The rendered-page regression passed, the template order was verified as Tag, Sort, User, Search, and `git diff --check` passed.
+
+
+### User
+fix the Mastodon timestamp assertion to expect UTC
+
+### Assistant outcome
+Updated the Mastodon posting regression assertion to expect the formatter's explicit `UTC` marker in the `Logged on` timestamp. The targeted test passed with `1 passed, 35 deselected`.
+# Chat Log
diff --git a/VIBE/PROMPTS.md b/VIBE/PROMPTS.md
index ed7602d..b38540c 100644
--- a/VIBE/PROMPTS.md
+++ b/VIBE/PROMPTS.md
@@ -1,3 +1,4 @@
+258. fix the Mastodon timestamp assertion to expect UTC
 # Prompt Log
 
 ## 2026-08-24
diff --git a/backend/tests/test_api.py b/backend/tests/test_api.py
index afa4a32..f4fb02a 100644
--- a/backend/tests/test_api.py
+++ b/backend/tests/test_api.py
@@ -899,7 +899,7 @@ def test_link_submission_posts_to_enabled_mastodon_plugin():
         assert received['path'] == '/api/v1/statuses'
         assert received['authorization'] == 'Bearer test-token'
         assert received['content_type'] == 'application/x-www-form-urlencoded'
-        assert received['body'] == {'status': ['From my #LinkLog:\n\nA useful page\n\nWorth sharing\n\nLogged on 2026 August 29 - 21:10 from: https://example.com/useful\n\n#python #web']}
+        assert received['body'] == {'status': ['From my #LinkLog:\n\nA useful page\n\nWorth sharing\n\nLogged on 2026 August 29 - 21:10 UTC from: https://example.com/useful\n\n#python #web']}
         posted_item = next(item for item in client.get('/api/public/feed/alice', headers=headers).json() if item['id'] == response.json()['id'])
         assert posted_item['mastodon_posted'] is True
     finally: