Jail Math

When will I get out? Jail time is a hard thing to add up in my head. It is as if we all need Math Tutors to help us figure out how much time we will spend in here.

I have a friend who spent 30 days in jail on the weekends, so he had 10 weekends to serve where he spent Friday and Saturday night in jail with each stay lasting 48 hours. At first glance I counted 10 times 2, or 20 days, but apparently I was wrong and it was 30 days. If I had a misdemeanor DUI and had six months to serve, then I would be out in three months.
As it is, I have 12 months to serve and I think I am supposed to serve every day of it. I just can't figure out the math.

My children will go to SCORE! so they can have a math tutor who can help them figure all of this out. I would not want them to be stuck and not know how to figure out the length of time for their sentence in jail.