All instructors have effective date fields.
Instructor effective date fields can be added via the Instructor Template.
If effective dates are defined, then an instructor will be filtered out of the "Assign Instructor" modal if the dates do not encompass the dates of the section (if section dates are not defined, then the dates of the term).
While an instructor can have an effective date field, Coursedog only stores one version of the instructor in the system. For institutions that have effective dating built into their SIS, when loading SIS data into Coursedog, only a single effective-dated version of the instructor can exist in Coursedog at any given time.