John recently discovered that he was being charged 2.1GB for Sunday, even though it was Saturday when he found out. What gives? Leo says it's likely based on UTC Universal Time Zone or Greenwich Mean Time. So that's why it's listed as tomorrow. But if you have had data used up when you're not using it, then someone may be "borrowing a cup of wifi" from you. You can also check your router's data logs. Call AT&T and tell them that you aren't using that much data.