Average based on multiple conditions - Average of workdays between two dates based on

Hi
My data sheet looks like this, (please see the attached sheet)



Problem:
I want to calculate AVERAGE workdays (number of days between “Date & Time Raised” and “Date & Time Closed”), based on following conditions,

Conditions:
1) Severity = “Prod Severity 4” OR “Development Severity 4”
2) Status = “Closed”
3) Month Closed = e.g. “Oct-2008”
4) Ignore errors (e.g. #VALUE!) while taking average of elapsed day

This datasheet is a dump from another database, the columns “Date & Time Raised” and “Date & Time Closed” may not be in date format. I use ‘dd/mm/yyyy’ format. I would like to ignore the time stamps while computing workdays.

Unable to insert any new columns to compute part-by-part. Please suggest a single cell formula which can be entered. An array formula taking all the above conditions would be great.

Thanks in advance.