Since "Offer" and "DateContacted" are fields, the fields cannot have a value of "IS NULL". So [Offer]="IS NULL" is an invalid test. If you wanted to test if a field is null, you would use the IsNull function: "ISNULL([FieldName])".
Because the two fields are date/time fields, I would try the IsDate() function.
For use in a query:
(using negative logic ---> Not IsDate([Offer]) )
Code:
Expr1: IIf(Not IsDate([Offer]),IIf(Not IsDate([DateContacted]),0,Date()-[DateContacted]+1),IIf(Not IsDate([DateContacted]),0,[Offer]-[DateContacted]+1))
I don't like to use negative logic, so here is positive the positive logic version:
(using negative logic ---> IsDate([Offer]) )
Code:
Expr2: IIf(IsDate([Offer]),IIf(IsDate([DateContacted]),[Offer]-[DateContacted]+1,0),IIf(IsDate([DateContacted]),Date()-[DateContacted]+1,0))