You've presented us with two different issues:
(1) Correctly interpreting the textfile date. For this, you'll need to provide more information (and examples).
(2) Manipulating business days (weekdays). Hopefully the following function will resolve that part of the problem:
Function SubtractWkDays(vardate As Variant, numdays As Integer, incl As Boolean)
'*******************************************
'Name: SubtractWkDays (Function)
'Purpose: Simple, non-formula method of
' subtracting weekdays from a given
' date
'Inputs: ? subtractwkdays("10/6/00", 300, false) --or--
' ? subtractwkdays(#10/6/00#, 300, false)
'Output: 8/15/1999
'Note: if incl = true then include vardate in the
' calculation, otherwise don't
'*******************************************
Dim thedate As Date, n As Integer
thedate = DateValue(vardate)
n = numdays
Do While n > 0
If WeekDay(thedate, 1) >= 2 And WeekDay(thedate, 1) <= 6 Then
n = n - 1
End If
thedate = thedate - 1
Loop
SubtractWkDays = thedate + IIf(incl = True, 1, 0)
End Function