Answer: The expiration date is set by the initial instructor certification a person receives. Topics that are added at a later date have the same expiration date as the initial expiration date.
For example: If your application for certification was approved on July 1, 2017, your first renewal date will be calculated four years from the next March 31st, so your renewal date will be March 31, 2022. If you add a specialized topic on July 1, 2019, the expiration date of both your General Instructor Certification and the specialized topic certification will remain March 31, 2022.