diff --git a/resources/prosody-plugins/mod_conference_duration.lua b/resources/prosody-plugins/mod_conference_duration.lua
index 28141e86ba..43f4511b59 100644
--- a/resources/prosody-plugins/mod_conference_duration.lua
+++ b/resources/prosody-plugins/mod_conference_duration.lua
@@ -25,7 +25,7 @@ function occupant_joined(event)
 
     if participant_count > 1 then
         if room.created_timestamp == nil then
-            room.created_timestamp = os.time() * 1000; -- Lua provides UTC time in seconds, so convert to milliseconds
+            room.created_timestamp = string.format('%i', os.time() * 1000); -- Lua provides UTC time in seconds, so convert to milliseconds
         end
     end
 end