Thanks for the replies.

The query works when I do not supply a criteria for the
calculate_working_days field.  I have checked for Null values in the date
fields and there doesnt seem to be any.  I have also tried changing the data
type being passed back from the function from Integer to Variant and Long.

I have used this function in the same way in the past and it normally works!!

The function is as follows:

Public Function Calculate_Working_Days(StartDate As Date, EndDate As Date)
As Integer

'--------------------------------------------------
'Calculates the correct amount of working days
'between two dates including bank holidays. Bank
'holidays are contained in the table zSYS_tblSpecialDays
'--------------------------------------------------

Dim dayCount As Integer
Dim wkendCount As Long '---- No of weekend days to be subtracted ----
Dim intDiff As Long  '---- Number of all days between 2 dates ----
Dim dbldiff As Double
Dim finaldiff As Long   '---- Final figure
Dim rstDays As Recordset
Dim SpecCount As Integer '---- Number of special days between the two dates
----
Dim DateRet1 As Date '---- Same date as Date2 ----
Dim date1 As Date
Dim date2 As Date
Dim nulldate As Integer

'Dim db As database

Set db = CurrentDb()
Set rstDays = db.OpenRecordset("zSYS_tblSpecialDays")
 
Dim rec As Long

DateRet1 = Format(DateRet1, "dd/mm/yyyy")
StartDate = Format(StartDate, "dd/mm/yyyy")

rec = rec + 1

   If StartDate <> Empty And EndDate <> Empty Then
       
       If StartDate < EndDate Then
       
           date1 = StartDate
           date2 = EndDate
           
       Else
           date1 = EndDate
           date2 = StartDate
       End If
           
   Else
       nulldate = 1
   End If
   
       
   If nulldate = 0 Then
   
       intDiff = date2 - date1
       
       wkendCount = 0
       SpecCount = 0
       
       Do Until date2 = date1
           
           dayCount = WeekDay(date2)
           
           If dayCount = 1 Or dayCount = 7 Then
               wkendCount = wkendCount + 1
           End If
           
       '---- Calculates amount of bank holidays between dates ----
       ' Uses daycount and checks Fridays, Mondays and Tuesdays
       
           If dayCount = 2 Or dayCount = 3 Or dayCount = 6 Then
           
               rstDays.MoveFirst
                         
               Do Until rstDays.EOF
                   If rstDays![Date] = date2 Then
                       SpecCount = SpecCount + 1
                   End If
                   
                   rstDays.MoveNext
               
               Loop
                   
               rstDays.MoveFirst
               
           End If
               
           date2 = date2 - 1
                             
         '  Debug.Print date1, date2, daycount, wkendCount, SpecCount
                           
       Loop
       
               
       '---- Final total of working days ----
                     
       If StartDate < EndDate Then
          finaldiff = intDiff - (wkendCount + SpecCount)
       Else
           dbldiff = 2 * (intDiff - (wkendCount + SpecCount))
           finaldiff = intDiff - (wkendCount + SpecCount) - dbldiff
       End If
             
   End If
       
   Calculate_Working_Days = finaldiff
   
   nulldate = 0

End Function