diff --git a/heat_tempest_plugin/common/test.py b/heat_tempest_plugin/common/test.py
index d719ad0..97d8d8c 100644
--- a/heat_tempest_plugin/common/test.py
+++ b/heat_tempest_plugin/common/test.py
@@ -60,6 +60,12 @@
     return False
 
 
+def isotime(at):
+    if at is None:
+        return None
+    return at.strftime('%Y-%m-%dT%H:%M:%SZ')
+
+
 def rand_name(name=''):
     randbits = six.text_type(random.randint(1, 0x7fffffff))
     if name: