Buffer Time doesnt works | WordPress.org

i just came through plugin and i was just testing it but Buffer Time Before Buffer Time After both fail to work.

as shown in picture above i want appointment to be 1hr and after each appointment there should be gap of 30min which means 8am-9am, 9:30am-10:30am, 11am-12am,12:30am-1:30am but buffer time doesnt work at all, and why buffer time is strict to 30minutes min why cant we go below 30min like 20min,15min,10min