diff --git a/b/cntoai/model_usage_admin_report.dspy b/b/cntoai/model_usage_admin_report.dspy
index 04e6861..bf0e068 100644
--- a/b/cntoai/model_usage_admin_report.dspy
+++ b/b/cntoai/model_usage_admin_report.dspy
@@ -307,7 +307,11 @@ def _aggregate_admin_summary(items, user_map, org_map):
         bucket['request_count'] += 1
         usage_time = item.get('usage_time')
         if usage_time:
-            bucket['last_usage_time'] = usage_time
+            if (
+                not bucket.get('last_usage_time')
+                or str(usage_time) > str(bucket['last_usage_time'])
+            ):
+                bucket['last_usage_time'] = usage_time
             if (
                 not bucket.get('first_usage_time')
                 or str(usage_time) < str(bucket['first_usage_time'])