Query Using Field From Current Open Form Using VBA Function

I am trying to resolve an issue of being able to use a query from many different forms where the query is dependent on the date selected in the current open form. Right now it calls the function getDate() which works fine. Unfortunatly the function is called before the form is fully loaded and the control I want to pass has no value/doesn't exist yet and I get a
"Runtime error 13 Type-Mismatch"

Here is the function code:

Function getDate() As String

If fIsLoaded("F_SupplierData") Then
getDate = Form_F_SupplierData.txtDate
Else
getDate = "01/01/1901"
End If

End Function

Function fIsLoaded(ByVal strFormName As String) As Integer
'Returns a 0 if form is not open or a -1 if Open
If SysCmd(acSysCmdGetObjectState, acForm, strFormName) <> 0 Then
If Forms(strFormName).CurrentView <> 0 Then
fIsLoaded = True
End If
End If
End Function


Is there a way to detect the state of the text field itself (if it is open, closed, dirty)? Or if anyone has a better way to tie a query to multiple forms?