Handling request times within certain business hours


I am looking for a way to calculate the number of days between when a row is added and when it is marked as "complete."

I currently have a column set up with the submission timestamp. I also have a "date completed" column. I want to exclude weekends and holidays which seems simple enough, but I would also like to set a criteria that if a row is added past a particular time (6pm), that day is not counted towards the turnaround time.

I was thinking of trying to figure out a formula that could maybe be added to another column where if the time on the original submission was past 6pm, it would return the next date, and then do the networkdays formula between that date and the completed date. I haven't been able to come up with such a formula that would do this however. I am also open to completely different solution if there are better ways to do this.

Thanks a lot.