add days using networkdays

I'm sure this could be done with a formula (or more efficiently in code), but here's a quick hack:

Function NextWorkDay(dat As Date, nDays As Long) As Date
    Dim iSgn As Long
    
    NextWorkDay = dat
    iSgn = Sgn(nDays)
    
    Do Until nDays = 0
        NextWorkDay = NextWorkDay + iSgn
        If Weekday(NextWorkDay, vbSaturday) > 2 Then nDays = nDays - iSgn
    Loop
End Function

You can use WORKDAY function,

=WORKDAY(A1,5)

where A1 contains your date. WORKDAY also allows you to specify a range of holiday dates to be excluded.

Humbug ...