Easy bit first. For the second Client, it is ReportDue = date(DateSeen) + 15
First client is a bit more tricky if you have to take public holidays into account. If not, and assuming Sat+Sun are non business days, then
switch
case dow(date(DateSeen)) = "Sat" : ReportDue = date(DateSeen) + 13
case dow(date(DateSeen)) = "Sun" : ReportDue = date(DateSeen) + 12
otherwise : ReportDue = date(DateSeen) + 14
endSwitch
If you need to take holidays into account then you will have to use a table which depends on the relevant country and increment ReportDue if a holiday falls between DateSeen and the above ReportDue date (and there may be more than one holiday!!)
(Tip: with a bit of fiddling, you can use MS Outlook to generate a suitable 'holiday' table)