Including working hours and more for average lead time

Hello all,

I'm trying to get the average lead time (including working hours and excluding holidays, weekends and leap years)  between a start date (creationDate) and end date (completionDate). Each record is registered in this format:

12-9-2016 15:01:00.

30-8-2019 13:23:00

etc.

I know DATADIFF exists but this doesn't get the result I need. How do I include parameters that only counts hours from 08:30 to 17:00 without weekends and holidays?