If your app sells an allowance, a number of credits a month, or a number of documents, or a number of anything. Whatever it is, you end up writing a balance somewhere, a reset when the period rolls over, a check before the expensive call, and a usage screen that has to agree with all of it. None of that is hard on its own. What gets you is that the pieces drift. The plan says a thousand a month, the reset runs on the first of the month, the subscription renews on the 18th, and the screen sums...