VBA Null Date Argument - vba

Question

I am trying to use a VBA UDF in access in an update query, but its not working. Here is my function:
Function final_forecast(status As String, _
Optional LDP As String, _
Optional fgpo_mode As String, _
Optional fgpo_start As Date, _
Optional fgpo_piw As Date, _
Optional asn_mode As String, _
Optional asn_piw As Date, _
Optional maker As Double, _
Optional origin As Double, _
Optional water As Double, _
Optional pod As Double, _
Optional transit As Double, _
Optional manual_piw As Date = 0, _
Optional delivery As Date = 0) As Date
Dim this_week, in_transit_piw, at_factory_piw, semi_final_piw, final_piw As Date
this_week = Date - Weekday(Date, 1) + 7
If status = "In Transit" Then
If Not delivery = 0 Then
in_transit_piw = delivery
ElseIf Not manual_piw = 0 Then
in_transit_piw = manual_piw
ElseIf asn_mode = "A" Then
in_transit_piw = asn_piw
Else:
in_transit_piw = DateAdd("d", nz(water, 0), asn_piw)
End If
ElseIf status = "At Factory" Then
If fgpo_mode = "A" Then
at_factory_piw = fgpo_piw
ElseIf LDP = "LDP" Then
at_factory_piw = DateAdd("d", nz(maker, 0) + nz(origin, 0) + nz(water, 0), fgpo_start)
Else:
at_factory_piw = DateAdd("d", nz(maker, 0) + nz(origin, 0) + nz(water, 0), fgpo_piw)
End If
End If
If in_transit_piw < this_week Then
semi_final_piw = this_week
ElseIf fgpo_mode = "A" Then
semi_final_piw = fgpo_piw
ElseIf at_factory_piw < DateAdd("d", 21 + nz(water, 0), Date) Then
If LDP = "LDP" Then
semi_final_piw = this_week
Else:
semi_final_piw = DateAdd("d", 28 + nz(maker, 0) + nz(origin, 0) + nz(water, 0), Date)
End If
Else: semi_final_piw = at_factory_piw
End If
If semi_final_piw < this_week Then
final_piw = this_week
Else:
final_piw = semi_final_piw
End If
final_forecast = final_piw
End Function
Here is a sample row that is throwing a "Compile Error: Expected Expression".
a = final_forecast("In Transit",,"B",#6/23/21#,#9/20/21#,"B",#8/11/21#,0,0,27,0,0,,)
debug.Print a
The last 2 arguments are empty date fields in my table. I have searched high and low for an answer. Can anyone help?
EDIT:
The update query looks like this:
UPDATE new_in_dc
SET
new_in_dc.[Final PIW] = final_forecast([new_in_dc].[In Transit Status],
[new_in_dc].[LDP],
[new_in_dc].[FGPO Mode Ship],
[new_in_dc].[FGPO Start Ship Dt],
[new_in_dc].[FGPO Prj In Wh Dt],
[new_in_dc].[Asn Mode Shp],
[new_in_dc].[Asn Prj In Wh Dt],
[new_in_dc].[FGPO Maker],
[new_in_dc].[Origin Port Delay],
[new_in_dc].[Water Delay],
[new_in_dc].[POD Delay],
[new_in_dc].[Transit Delay],
[new_in_dc].[Manual PIW],
[new_in_dc].[Delivery Date]);

Answer 1

You can (should) remove those last two commas in the list of arguments to final_forecast
When passing arguments by position, you only need the commas in the middle of the arguments list: if at the end they should be left off.
What happens if you try to run a query like this in your database?
select final_forecast(t.[In Transit Status],
t.[LDP],
t.[FGPO Mode Ship],
t.[FGPO Start Ship Dt],
t.[FGPO Prj In Wh Dt],
t.[Asn Mode Shp],
t.[Asn Prj In Wh Dt],
t.[FGPO Maker],
t.[Origin Port Delay],
t.[Water Delay],
t.[POD Delay],
t.[Transit Delay],
t.[Manual PIW],
t.[Delivery Date])
from new_in_dc t

Answer 2

Remove to two trailing commas and it will run:
a = final_forecast("In Transit",,"B",#6/23/21#,#9/20/21#,"B",#8/11/21#,0,0,27,0,0)
If you use the function in a query whery you may have Null values for some records for a required field, use Nz to provide a valid value, for example for the last two arguments, the date of today:
a:final_forecast("In Transit",Nz([FieldA],"N/A"),"B",#6/23/21#,#9/20/21#,"B",#8/11/21#,0,0,27,0,0,Nz([FieldB],Date()),Nz([FieldC],Date()))

Answer 3

I had to wrap everything in Nz([Field],0) to get the update query to get my UDF to work.
UPDATE new_in_dc SET new_in_dc.[Final PIW] = final_forecast([new_in_dc].[In Transit Status],nz([new_in_dc].[LDP],0),nz([new_in_dc].[FGPO Mode Ship],0),nz([new_in_dc].[FGPO Start Ship Dt],0),nz([new_in_dc].[FGPO Prj In Wh Dt],0),nz([new_in_dc].[Asn Mode Shp],0),nz([new_in_dc].[Asn Prj In Wh Dt],0),nz([new_in_dc].[FGPO Maker],0),nz([new_in_dc].[Origin Port Delay],0),nz([new_in_dc].[Water Delay],0),nz([new_in_dc].[POD Delay],0),nz([new_in_dc].[Transit Delay],0),nz([new_in_dc].[Manual PIW],0),nz([new_in_dc].[Delivery Date],0));