diff --git a/ceilometer/storage/impl_mongodb.py b/ceilometer/storage/impl_mongodb.py
index db3dd581d..98af16e92 100644
--- a/ceilometer/storage/impl_mongodb.py
+++ b/ceilometer/storage/impl_mongodb.py
@@ -330,7 +330,10 @@ class Connection(pymongo_base.Connection):
                res.duration_start = values[i].duration_start;
             if ( values[i].duration_end > res.duration_end )
                res.duration_end = values[i].duration_end;
-        }
+            if ( values[i].period_start < res.period_start )
+               res.period_start = values[i].period_start;
+            if ( values[i].period_end > res.period_end )
+               res.period_end = values[i].period_end;        }
         return res;
     }
     """)
@@ -339,8 +342,7 @@ class Connection(pymongo_base.Connection):
     function (key, value) {
         %(aggregate_val)s
         value.duration = (value.duration_end - value.duration_start) / 1000;
-        value.period = NumberInt((value.period_end - value.period_start)
-                                  / 1000);
+        value.period = NumberInt(%(period)d);
         return value;
     }""")
 
@@ -894,7 +896,8 @@ class Connection(pymongo_base.Connection):
         )
         reduce_stats = self.REDUCE_STATS % reduce_params
 
-        finalize_params = dict(aggregate_val=sub('finalize', aggregate))
+        finalize_params = dict(aggregate_val=sub('finalize', aggregate),
+                               period=(period if period else 0))
         finalize_stats = self.FINALIZE_STATS % finalize_params
 
         results = self.db.meter.map_reduce(
diff --git a/ceilometer/tests/storage/test_storage_scenarios.py b/ceilometer/tests/storage/test_storage_scenarios.py
index 77ad85619..1aec91cc1 100644
--- a/ceilometer/tests/storage/test_storage_scenarios.py
+++ b/ceilometer/tests/storage/test_storage_scenarios.py
@@ -1182,6 +1182,10 @@ class StatisticsTest(DBTestBase,
         self.assertEqual(10, results.max)
         self.assertEqual(27, results.sum)
         self.assertEqual(9, results.avg)
+        self.assertEqual(datetime.datetime(2012, 9, 25, 10, 30),
+                         results.period_start)
+        self.assertEqual(datetime.datetime(2012, 9, 25, 12, 32),
+                         results.period_end)
 
     def test_by_user(self):
         f = storage.SampleFilter(