Where does the 'Effective Date' on forms pull from?

If a form designer form has a field with the ID ReportTime, then that is what is used for the effective date. It means that the form is looking at the ReportTime field (label "Commence Transfer" in the example below) for the date rather than the activities themselves:

For Standard Forms, a hidden date field is used which gets set to PS activity time if it exists, otherwise, it is the earliest activity time.