I have a iPad with a 2 GB plan. My billing cycle starts the 3rd of every month. On March 2nd, I only had used 800 MB of data so I figured downloading a 287.5 MB file would be okay as my usage data would reset the next day. Well stupid me, the 287.5 MB data counted to the March 3rd to April 2nd billing cycle, not the Feb 3nd to March 2nd billing cycle, despite that AT&T knows that the data was used on March 2nd.

I went back and checked my older bills and saw that there was data used from the 2nd of each month on all my bills. Sometimes data usage would be from the 2nd of the month to the next 2nd, sometimes from the 1st to the 1st of the following month.

I called AT&T to complain since AT&T is counting data outside the billing cycle as being part of the billing cycle. I was told that since it takes 24 to 48 hours for data usage to show up in their system, rather then go back and apply data used during the previous billing cycle to that cycle, they simply apply it to the current billing cycle. Despite the fact that the data wasn't used in that cycle.

So basically say your billing date is the 3rd of every month, like mine. Data from the 2nd of the month (and possibly the 1st) will be applied to that billing cycle. That's not correct and that shouldn't be happening, but it does.