Order Of Events Help On Code

I have a start and end date fields plus a txt box called wDays.
I'm trying to make sure the user puts in both dates but end cant be earlier than start and vs versa....
If not provide a message as to which is missing....
If alls well do the calulation with the module "GetWorkDays"
I f everything goes bad, dont save the record at all!

I have this behind a cmdButton. Used to unhide the wDays field because it displays #error otherwise...and to do the calculation... with the BeforeUpdate
Private Sub Command8_Click()
If IsNull(Me.sDate) Then
MsgBox "Please enter a Start Date"
ElseIf IsNull(Me.eDate) Then
MsgBox "Please enter a End Date"
Else
If Me.eDate < Me.sDate Then
MsgBox "End Date cannot be earlier than the Start Date"
Exit Sub
End If
End If
Me.Requery
End Sub

Private Sub Form_BeforeUpdate(Cancel As Integer)
Dim myDays As Integer
If IsNull(Me.eDate) Or IsNull(Me.sDate) Then
Me.wDays.Visible = False
Else
Me.wDays.Visible = True
myDays = GetWorkDays([sDate], [eDate])
Me.wDays.Value = myDays
End If
End Sub


Any help would be really appreaciated because I have tried many combonations of code placement to get it working correctly....