MYSQL query (maybe take just 10 min.)

I want to calculate via MariaDB query a date of recurring subscription based on this information:
For Example:
start_date:01.01.2019
period_of_validity: 4 (Months)
regular_finish_date:01.05.2019, 01.09.2019, 01.01.2020, "01.05.2020"
finished_by_user_at:
membership_will_finished_at:
Case1:
When finished_by_user_at is NULL (It means, the user doesnt cancel his subscription) the regular_finish_date should be update automatically after the interval was reached. As we can see in the example above, the date it should show now is 01.05.2020.
Wo can help me with this Task? No php, java or something else is possible. Only MYSQL query. Thanks