How to add attributes as a reference to another Attribute for calculation using EL in HSDL

Summary:

How to do calculation for the Duration by taking Start Date and End Date attribute in Absence

Content (please ensure you mask any confidential information):

Hi Team

I am trying to calculate Start Date Duration by taking difference between the reference attribute Start Date and End Date using expression in HSDL , but it is not working because the reference attributes(Start Date , End Date) are in date format but the Start Date Duration is in number format. Can someone help me or suggest the better to achieve this using reference attribute calculation. I need to create an HSDL for Absence Entry Correction.