Odd. Over the course of my education I went to 3 different universities in the EU. Classes/lectures/labs, they all started at the advertised time and I’ve never encountered a concept of “c.t” or “s.t”. Not a formal one anyway. People “talked” about the “academic 15 minutes” but like it was a thing of the past.
I've had one side of the same university campus observing the academic 15 minutes, while in one course on the other side did not... after the lunch break. So at 13:30 we started walking towards the other side of the campus (the class was "scheduled" at 13:30), but did not receive a warm welcome 10 minutes later, because the lecturer had already started at the "scheduled" time.