I can confirm that the system calculates the days of validity starting from the day the user self-enrolled into the course (placed on the course waiting list). As-soon-as the user enrolls and is added to the waitlist, the system calculates the days of validity and would set an Active From / Active Until date in the course enrollment area.
 
If this is not in line with your requirements, I would perhaps suggest editing the days of validity from “Start calculation from the enrollment date” to “Start calculation from the first access date”