diff --git a/ceilometer/tests/pipeline_base.py b/ceilometer/tests/pipeline_base.py
index 5359486ba..b7a3f4814 100644
--- a/ceilometer/tests/pipeline_base.py
+++ b/ceilometer/tests/pipeline_base.py
@@ -1205,6 +1205,15 @@ class BasePipelineTestCase(test.BaseTestCase):
         self.assertEqual(expected_length, len(publisher.samples))
         return sorted(publisher.samples, key=lambda s: s.volume)
 
+    def test_aggregator_input_validation(self):
+        aggregator = conversions.AggregatorTransformer("1", "15", None,
+                                                       None, None)
+        self.assertEqual(1, aggregator.size)
+        self.assertEqual(15, aggregator.retention_time)
+
+        self.assertRaises(ValueError, conversions.AggregatorTransformer,
+                          "abc", "cde", None, None, None)
+
     def test_aggregator_metadata(self):
         for conf, expected_version in [('last', '2.0'), ('first', '1.0')]:
             samples = self._do_test_aggregator({
diff --git a/ceilometer/transformer/conversions.py b/ceilometer/transformer/conversions.py
index 543432b55..8954608e3 100644
--- a/ceilometer/transformer/conversions.py
+++ b/ceilometer/transformer/conversions.py
@@ -188,8 +188,8 @@ class AggregatorTransformer(ScalingTransformer):
                  **kwargs):
         super(AggregatorTransformer, self).__init__(**kwargs)
         self.samples = {}
-        self.size = size
-        self.retention_time = retention_time
+        self.size = int(size) if size else None
+        self.retention_time = float(retention_time) if retention_time else None
         self.initial_timestamp = None
         self.aggregated_samples = 0
 
@@ -225,8 +225,7 @@ class AggregatorTransformer(ScalingTransformer):
 
     def handle_sample(self, context, sample):
         if not self.initial_timestamp:
-            self.initial_timestamp = timeutils.parse_strtime(
-                sample.timestamp)
+            self.initial_timestamp = timeutils.parse_isotime(sample.timestamp)
 
         self.aggregated_samples += 1
         key = self._get_unique_key(sample)