C.t. is short for the Latin expression “cum tempore” (with time). If your read “c.t.” next to the time for a lecture or seminar in the course programme, this means that it starts 15 minutes later (8 am ct = 8:15 am). If the course programme says a lecture or seminar starts “s.t.” this stands for “sine tempore” (without time). “S.t.” means that you should arrive punctually at the time given.