Display Result Of Date Function In Label

I am trying to display the result of a custom VBA function as a label on a userform. The function is called and takes arguments from three text boxes on the userform. I attached it to the exit parameter. (This may be part of the problem; I'm not sure).

There seem to be two problems, first all the text boxes don't always contain data, but because the arguments are not optional, I get a type mismatch error when the code runs (I tried switching some of them to optional, but it didn't help as you can see from my 'commented code).

The other problem is that even when all the arguments seem to be met (i.e. the text boxes all have data), I still get a compile error that the argument is not optional. This only happens if I try and pass the result to some aspect of the userform. As you can see from the last coded line in the custom function, if I assign it to a range on the worksheet, it functions fine.

Private Sub txtStartDate_Exit(ByVal Cancel As MSForms.ReturnBoolean)
Dim StartDate As Date
Dim EndDate As Date
If IsDate(Me.txtStartDate.Value) Then
StartDate = Me.txtStartDate.Value
Me.txtStartDate.Value = Format(StartDate, "m/d/yyyy")
Else: MsgBox "Please enter a date"
End If.............