Duration in Date rule in CRM

Hello Friends,

I have a requirement to calculate and display the Duration.

The ‘Open Duration’ can only be calculated once the Dispute Status at Header Level is set to ‘In Process’ and its figure must be based by the calculated difference of ‘Notification Receipt’ minus ‘Notification In Process’ where both are ‘Date Types’.

We need to change the same in XML file ? in date rule of date types ?

I need code how to read and change the values