Sorry to be dense, but now I'm lost. The record source for the form is the query that has the date/time field "WorkdayRecd". The three due date fields in the query are based on that field using your PlusWorkdays module. They are:
2DayDue: PlusWorkdays([WorkdayRecd],2)
27DayDue: PlusWorkdays([WorkdayRecd],27)
30DayDue: PlusWorkdays([WorkdayRecd],30)
These are the Control Source for the three due date fields in the form. By "calculations," did you mean the due date fields?
When you posted, "You can use IIF( IsError([YourField]) , "", [YourField]) in the ControlSource of the control," was that not supposed to be in the Control Source of each due date field of the form?