[LukeShu]

When my cell plan was with T-mobile, I was on a pre-paid plan. My bill was due once a month, so I set up monthly automatic billing. Well, the system that enabled or disabled my service depending on if I had paid considered that to be "every 30 days". The system that handled the automatic payments considered that to be the same day every month (IIRC, it charged my card on the 18th of every month; when I set it up my bill was due on the 20th). A couple of 31-day months later, and I lose service for a day because the automatic payments didn't keep up with the bill being due.