There are a few things that could impact your design when it comes to tracking accrual. These include:
* The type of interest are you wanting to track? Compound, Simple, Rule of 78 etc.
* How the annual percentage rate is to be calculated ie 366/365, 365/365, 90 day bank bill rates
* Are you looking at accruing interest for a set periond, and then capitalising it on a specific date?
* If the balance can be changed, do you need to handle backdated transactions?
* If yes to the previous question, what legal obligations do you have to reimburse pre-charged interest or request short-falls?
I'm probably going over the top, but without knowing what the intended audience is, I thought I would just throw up a few questions. There are plenty more, but you can let me know if I'm heading off in the wrong direction.
Cheers
SteveA