Thanks for your quick reply

sample data pasted below:
Request Received Date & time : 7/31/2015 21:15:00 PM
Request Completion date & time : 8/3/15 9:56 PM
need to complete the request with in 4 business hours. Business hours start from 17.00 to 02.00.while calculating it shouldn't include weekends and Holidays.
Please provide formula

Thanks in Advance
Srinath