Date diff in roll up format

Hi All, I am facing a trouble to get roll up the date diff in tableau report .

useCase:

First i need to find difference of start and end date ( which i have done with the help of date_part function)

once i get the difference then i need to use in aggregation like average number of hours and then show that hours in form of days and hour or month ,days and hour for e.g if average difference is 100 hour the it should how ( 4d ,4h )