diff --git a/backend/services/vaults.py b/backend/services/vaults.py
index 8df2672..1303367 100644
--- a/backend/services/vaults.py
+++ b/backend/services/vaults.py
@@ -7,6 +7,7 @@ Single source of truth consumed by both the REST routes (``/api/vaults``,
 
 from __future__ import annotations
 
+from datetime import UTC
 from pathlib import Path
 from typing import Any
 
@@ -49,7 +50,7 @@ def get_vault_root(vault_name: str) -> Path:
 
 def browse_directory(vault_name: str, path: str = "") -> dict[str, Any]:
     """Return the direct children of a vault directory (directories first)."""
-    from datetime import datetime, timezone
+    from datetime import datetime
 
     from backend.indexer import SUPPORTED_EXTENSIONS
     from backend.vault_settings import get_vault_setting
@@ -98,7 +99,7 @@ def browse_directory(vault_name: str, path: str = "") -> dict[str, Any]:
                     "type": "file",
                     "size": stat.st_size,
                     "modified": stat.st_mtime,
-                    "modified_iso": datetime.fromtimestamp(stat.st_mtime, tz=timezone.utc).isoformat(),
+                    "modified_iso": datetime.fromtimestamp(stat.st_mtime, tz=UTC).isoformat(),
                     "extension": entry.suffix.lower(),
                 })
     except PermissionError:
@@ -156,7 +157,7 @@ def list_all_files(
         ServiceError: ``not_found`` (404) if the directory does not exist,
             ``permission_denied`` (403) if it cannot be read.
     """
-    from datetime import datetime, timezone
+    from datetime import datetime
 
     from backend.indexer import IGNORED_DIRS
 
@@ -215,7 +216,7 @@ def list_all_files(
                 "vault": vault_name,
                 "size": stat.st_size,
                 "modified": stat.st_mtime,
-                "modified_iso": datetime.fromtimestamp(stat.st_mtime, tz=timezone.utc).isoformat(),
+                "modified_iso": datetime.fromtimestamp(stat.st_mtime, tz=UTC).isoformat(),
                 "extension": ext.lstrip(".") if ext else "",
                 "tags": _indexed_tags(vault_name, rel_path),
             }