Can someone explain the 7 min grace period clocking in to me?

I didn’t know about this today. That our time clock rounds.. For example if I work 8:30am to 5:00pm, I can arrive work at 8:23am or 8:37am but still be considered at work at 8:00am right?

What if my lunch is at 12:00–12:30pm, and I choose to take only a 15 minute break…then how would clocking in round it? Does that mean I can leave 15 min earlier than scheduled time like at 4:45??I get to leave early if I shorten my lunch.