diff --git a/examples/cull-idle/cull_idle_servers.py b/examples/cull-idle/cull_idle_servers.py
index 656d8a0f..258832e5 100755
--- a/examples/cull-idle/cull_idle_servers.py
+++ b/examples/cull-idle/cull_idle_servers.py
@@ -75,7 +75,7 @@ def format_td(td):
     seconds = seconds % 3600
     m = seconds // 60
     seconds = seconds % 60
-    return f"{h:02}:{m:02}:{seconds:02}"
+    return "{h:02}:{m:02}:{seconds:02}".format(h=h, m=m, seconds=seconds)
 
 
 @coroutine