In my dojo the starting times are clearly stated and posted. My assumption is that most students will arrive in plenty of time to be on the mat at the proper starting time.
I remember Kobayashi, Sensei, frowning at me on occasion, not because I arrived after the start of class, but because I arrived with barely enough time to get changed into my dogi and on the mat. He felt that if you were rushed, even if you were on time, your mind was not in the proper frame.
Even today, I find myself rushing needlessly at times. This is my major character flaw (although I am certain my wife might think of others) in relation to time.
As an instructor, I do not comment on late arriving students. I start class on time and end on time, that is part of my responsibility. I do require that all late arriving students complete a full set of warm-ups and Aiki Taiso before I will allow them to train.
Timeliness in training begins by leaving early, allowing for traffic, and arriving far enough in advance of class that one can sit a slow down from the frantic pace of the day.
Just a different perspective.