Hi Rambabu,
 
Daniel here from customer.io - Thanks for reaching out! I understand you need some clarification on the start_ts & end_ts timestamp parameters in the List Messages App API Call.
 
These two date parameters are indeed based on the date a message was created and can be used to filter results for messages created within a specific time period. From there, you can use the metric parameter to include specific metric information for these messages specified in your time period selected.
 
On a side note, our team is currently reviewing this documentation as the created value for the metric parameter doesn't seem to be supported at this stage.
 
Otherwise, I hope this has helped out!