MS Access db, VBA form dynamic Default value on condition - ms-access

Question

I'm using form below and not sure if I can implement this logic:
Start Date = Now();
If Expedited then Exp_Date = Start_Date + 72 hours
else Exp_Date = Start_Date + 30 days.
I used this expression for default but looks like at time of evaluations it doen'st care about value in ExpeditedYN.
=IIf([ExpYN]=True,DateAdd("h",72,Now()),DateAdd("d",30,Date()))
Ideally I'd like to flip Exp.Date default clicking on Expedited box, is this available in VB. Tried DLookUp (D --> Dynamic??) and it doesn't work
DLookUp(IIf([ExpYN]=True,DateAdd("h",72,Now()),DateAdd("d",30,Date())))
Tx
Dai

Answer 1

Using IIf or DLookup to refer to another field on the same record in a Default Value isn't going to get you anywhere, because Access only asks Default Value what value to fill in when the new record is first initiated. That means it's going to be based on the default values of the other fields, and therefore will not respond to any user input.
I'd suggest that you keep the Default Value of both StartDate and DeadlineDate simple, similar to how you already are:
StartDate's Default Value: Now()
DeadlineDate's Default Value: DateAdd("d",30,Now())
And then change it in code if and when Expedited is changed. There's a few ways of doing this, I'd probably put it in the Before Update event of the checkbox, so it runs without having to move out of the record. You could also base it on the StartDate field if you might want to be able to change the record later on. You could build it with Access' macro builder, but it's easier to show you VBA here:
If ExpYN Then
DeadlineDate = DateAdd("h", 72, StartDate)
Else
DeadlineDate = DateAdd("d", 30, StartDate)
End If
If you want the expedition process to be based on the moment the button is clicked instead of the DateTime data in StartDate, just change StartDate (or whatever that field is called) to Now().

Answer 2

DefaultValue triggers only once - when record is first initiated by input into any field. If the calculation is based on value of Expedited, the DLookup should not work because the record is not yet committed to table. Record is committed when: 1. close table/query/form or 2. move to another record or 3. run code to save. If you want to programmatically modify DeadlineDate value based on Expedited value, then code needs to be in Expedited AfterUpdate event and abandon the DefaultValue approach.
Me!DeadlineDate = IIf(Me!ExpYN, DateAdd("h",72,Now()), DateAdd("d",30,Now()))