diff --git a/backend/services/usage_metrics_service.py b/backend/services/usage_metrics_service.py
index 8b1e09ba6..af9dafaa7 100644
--- a/backend/services/usage_metrics_service.py
+++ b/backend/services/usage_metrics_service.py
@@ -4,7 +4,7 @@
 import math
 import os
 from typing import Any, Optional
-from datetime import datetime
+from datetime import datetime, timezone
 import aiohttp
 from loguru import logger
 
@@ -464,7 +464,7 @@ def _safe_int(self, value: Any) -> int:
     def _format_created_at(self, created_at) -> str:
         """Format created_at to ISO8601 string."""
         if created_at is None:
-            return datetime.utcnow().isoformat()
+            return datetime.now(timezone.utc).isoformat()
 
         if isinstance(created_at, datetime):
             return created_at.isoformat()
diff --git a/tests/unit/test_usage_metrics_service.py b/tests/unit/test_usage_metrics_service.py
index f670b12d5..00e7b9305 100644
--- a/tests/unit/test_usage_metrics_service.py
+++ b/tests/unit/test_usage_metrics_service.py
@@ -378,3 +378,18 @@ def test_extract_llm_calls_falls_back_to_primary_without_token_metrics():
             "costUsd": 0,
         }
     ]
+
+
+def test_format_created_at_returns_utc_isoformat():
+    service = UsageMetricsService()
+    result = service._format_created_at(None)
+    assert "+" in result  # timezone-aware ISO string includes offset like "+00:00"
+    assert "Z" not in result  # should use +00:00, not Z suffix
+
+
+def test_format_created_at_returns_original_datetime_isoformat():
+    from datetime import datetime, timezone
+    service = UsageMetricsService()
+    dt = datetime(2024, 1, 1, 12, 0, 0, tzinfo=timezone.utc)
+    result = service._format_created_at(dt)
+    assert result == "2024-01-01T12:00:00+00:00"