Aight, thx.

It's still weird that they switch the way of telling a story. It's about how some people spend their time. The narrator in the book is like:
At 6:15, they do X, at 9:23, they do Y. Most of the time, they were using "в" for anything up to the 31st min. (excl. 31). The only exception was the 6:22 case, where they just omitted "в".

Based on your explanation, it must be like. "At 2:14, they go to the club (using "в"). It's 6:22 now, time to return home...(excluding "в")."
Or they just screwed up the grammar here...

Well, I wish I were wrong w/ my hearing. But it's actually a written story in the book.