A similar concept has been used for university lectures to start 15 minutes after the full hour. This was due to nobody having watches and everything being synced to the church bells ringing at the full hour. Then you had 15 minutes to get to the next lecture (in another building). Starting time was given as ct (cum tempore, with time) or st (sine tempore, without time). Usually only st was marked as everybody assumed the 15 minutes delay.

https://en.wikipedia.org/wiki/Academic_quarter_(class_timing...