diff --git a/metrik/targets/mongo.py b/metrik/targets/mongo.py
index f941291..d6f18a1 100644
--- a/metrik/targets/mongo.py
+++ b/metrik/targets/mongo.py
@@ -38,8 +38,7 @@ class MongoTarget(Target):
         # Because MongoDB isn't microsecond-accurate, we need to set the
         # microseconds to 0 to ensure consistency
         present_deref = present if present is not None else datetime.now()
-        present_deref.replace(microsecond=0)
-        id_dict['_created_at'] = present_deref
+        id_dict['_created_at'] = present_deref.replace(microsecond=0)
 
         with self.get_db() as db:
             return db[self.collection].insert_one(id_dict).inserted_id
diff --git a/test/targets/test_mongo_target.py b/test/targets/test_mongo_target.py
index 9814147..693f6ed 100644
--- a/test/targets/test_mongo_target.py
+++ b/test/targets/test_mongo_target.py
@@ -54,8 +54,8 @@ class MongoTargetTest(MongoTest):
         r = t.retrieve()
         self.assertGreaterEqual(present, r['_created_at'])
 
-        one_second_past = present - timedelta(seconds=1)
+        second_prior = (present - timedelta(seconds=1)).replace(microsecond=0)
         t = MongoTarget('test_collection', 1235)
-        t.persist({}, present=one_second_past)
+        t.persist({}, present=second_prior)
         r = t.retrieve()
-        self.assertEqual(one_second_past, r['_created_at'])
\ No newline at end of file
+        self.assertEqual(second_prior, r['_created_at'])
\ No newline at end of file