Default difference of Time entered in two different of Document of Records fields in a third Field

Summary:

Customer: Gulf Drug L.L.C, Approved UC:1 We have a requirement to capture the fields Overtime From Date and Time and Overtime To Date & Time in Document Records Descriptive Flexfields: Context Sensitive Flexfields. We want to achieve the below requirement through Autocomplete Rules: To get difference of time between Overtime To Date and Time and Overtime From Date and store in another field i.e. Hours Applied

Content (please ensure you mask any confidential information):

Currently, we are capturing Overtime From Date & Time in one field and Overtime To Date & Time in another field in a Run time. We want to default only time difference in a third DFF i.e. New Hours Applied.