diff --git a/backend/indexer.py b/backend/indexer.py
index d5ae560..5f8cf71 100644
--- a/backend/indexer.py
+++ b/backend/indexer.py
@@ -5,7 +5,7 @@ import os
 import re
 import threading
 from collections.abc import Callable
-from datetime import datetime, timezone
+from datetime import UTC, datetime
 from pathlib import Path
 from typing import Any
 
@@ -581,7 +581,7 @@ def _scan_vault(
             try:
                 relative = fpath.relative_to(vault_root)
                 stat = fpath.stat()
-                modified = datetime.fromtimestamp(stat.st_mtime, tz=timezone.utc).isoformat()
+                modified = datetime.fromtimestamp(stat.st_mtime, tz=UTC).isoformat()
 
                 # #86 differential scan: reuse the previous entry when neither
                 # size nor mtime changed — skips the disk read + parse below.
@@ -866,7 +866,7 @@ async def build_index(progress_callback=None) -> None:
 
     # Record timestamp of full index rebuild
     global _last_full_index_ts
-    _last_full_index_ts = datetime.now(timezone.utc).isoformat()
+    _last_full_index_ts = datetime.now(UTC).isoformat()
 
     # Build attachment index
     from backend.attachment_indexer import build_attachment_index
@@ -1033,7 +1033,7 @@ def _index_single_file_sync(vault_name: str, vault_path: str, file_path: str, va
             return None
 
         stat = fpath.stat()
-        modified = datetime.fromtimestamp(stat.st_mtime, tz=timezone.utc).isoformat()
+        modified = datetime.fromtimestamp(stat.st_mtime, tz=UTC).isoformat()
 
         # PDF handling — binary, must go through pdf_reader (same as _scan_vault)
         tags: list[str] = []