"ByRef argument type mismatch" Error for WeekNum(Comp_Date)
Help please...

Sub TAT()
Dim Effect_Recd_Date, Effect_Recd_Time As Date
Dim Recd_Date, Recd_Time, Comp_Date, Comp_Time, Shift_Start, Shift_End As Date
Dim Week_End As Date
R = 6
C = 35
Shift_Start = Range("D2").Value
Shift_End = Range("H2").Value
Week_End = DateSerial(2005, 1, 10) + (2 * Shift_Start) - DateSerial(2005, 1, 7) - Shift_End
Do
Recd_Date = Cells(R, C).Value
Recd_Time = Cells(R, C + 1).Value
'Calculation of Effective Received Time
If ((Recd_Time < Shift_End) Or ((Recd_Time >= Shift_Start) And (Recd_Time < 1))) Then
Effect_Recd_Time = Recd_Time
Else: Effect_Recd_Time = Shift_Start
End If
'Calculation of Effective Received Date
If ((Recd_Time <> Effect_Recd_Time) And (Weekday(Recd_Date, vbMonday) > 5)) Then
Effect_Recd_Date = Recd_Date + 8 - Weekday(Recd_Date, vbMonday)
Else: Effect_Recd_Date = Recd_Date
End If
Cells(R, C + 2).Value = Effect_Recd_Date
Cells(R, C + 3).Value = Effect_Recd_Time
'TAT Hour Calculation
Comp_Date = Cells(R, C - 11).Value
Comp_Time = Cells(R, C - 10).Value
If (WeekNum(Comp_Date) <> WeekNum(Effect_Recd_Date)) Then
TAT_Hour = Comp_Date + Comp_Time - Recd_Date - Recd_Time
End If

R = R + 1
Loop
'INT((X6+Y6-AA6-AB6)*24)-IF(WEEKNUM(X6)<>WEEKNUM(AA6),(WEEKNUM(X6)-WEEKNUM(AA6))*54.5,0)

End Sub

Function WeekNum(WeekDate As Date)
WeekNum = Round((DateSerial(Year(WeekDate), Month(WeekDate), Day(WeekDate)) _
- DateSerial(Year(WeekDate), 1, 0)) / 7, 0)
End Function