Supplier schedule past due quantity shown based on Release Need by date or Promise

Hi Friends,

Supplier Schedule workbench past due quantity will be shown on basis of PO Release shipment need by date or Promise date.

for Example: Item: ABC quantity# 31 Need by date: 12-Jun-2017 Promise date: 07-Jul-2017 but past due quantity showing as on 7th Jul 2017 here its considering Promise date.

for item: XYZ quantity :58 need to by date 25-Jun-2017 Promise date: 05-Jul-2017 but past due quantity showing based on need by date.

so bit confused what date basis past due would be shown.

Thanks

Laksh