diff --git a/doc/api_samples/os-aggregates/aggregates-metadata-post-resp.json b/doc/api_samples/os-aggregates/aggregates-metadata-post-resp.json
index 83bf953f0480..bcfeacd8f314 100644
--- a/doc/api_samples/os-aggregates/aggregates-metadata-post-resp.json
+++ b/doc/api_samples/os-aggregates/aggregates-metadata-post-resp.json
@@ -11,6 +11,6 @@
             "key": "value"
         },
         "name": "name",
-        "updated_at": null
+        "updated_at": "2013-08-18T12:17:55.986540"
     }
 }
diff --git a/nova/compute/api.py b/nova/compute/api.py
index 961611752090..7f2c2a5623de 100644
--- a/nova/compute/api.py
+++ b/nova/compute/api.py
@@ -3670,6 +3670,7 @@ class AggregateAPI(base.Base):
                                   action_name=AGGREGATE_ACTION_UPDATE)
         if values:
             aggregate.update_metadata(values)
+            aggregate.updated_at = timeutils.utcnow()
         self.scheduler_client.update_aggregates(context, [aggregate])
         # If updated values include availability_zones, then the cache
         # which stored availability_zones and host need to be reset
@@ -3689,6 +3690,7 @@ class AggregateAPI(base.Base):
         # which stored availability_zones and host need to be reset
         if metadata and metadata.get('availability_zone'):
             availability_zones.reset_cache()
+        aggregate.updated_at = timeutils.utcnow()
         return aggregate
 
     @wrap_exception()
diff --git a/nova/tests/functional/api_sample_tests/api_samples/os-aggregates/aggregates-metadata-post-resp.json.tpl b/nova/tests/functional/api_sample_tests/api_samples/os-aggregates/aggregates-metadata-post-resp.json.tpl
index b15c40fa5dc9..98bf8bb6322c 100644
--- a/nova/tests/functional/api_sample_tests/api_samples/os-aggregates/aggregates-metadata-post-resp.json.tpl
+++ b/nova/tests/functional/api_sample_tests/api_samples/os-aggregates/aggregates-metadata-post-resp.json.tpl
@@ -11,6 +11,6 @@
             "key": "value"
         },
         "name": "name",
-        "updated_at": null
+        "updated_at": %(strtime)s
     }
 }