The thing you MOST need to remember is that you're not an accountant. Not only do you not need to do the math yourself, you MUST NOT DO THE MATH YOURSELF. Talk to the people who do it, ask them how they do it there, and program based on their methods. They will give you the data, and they will give you the numbers that you should get FROM the data. It's your job to make programs that will replicate their methods exactly, so that when they're reconciling, everything comes together flawlessly.
Financials programming is very methodical, but there are many different methods, and different places will use different combinations of methods. How they calculate their accounting periods, how they write off debt and when they accrue various things. GL requirements, all that crap, it's all dictated by the company. Find the math guys (and there will be Access/Excel ninja accountant math guys, who can be a huge resource or a terrible nemesis), make friends with them, and try to figure out how to make their crazy methods into clean applications.
There are usually very strict controls put in place around programmers...As a general rule, you never want to have a single programmer who can "close the loop" which is to say, that you can't have the same guy dealing with incoming and outgoing money, because those people are theft waiting to happen. If you have the misfortune to be that guy (I am that guy) you need to adhere to the strictest propriety whenever you deal with money or it will come back to bite you in the worst possible way.
I once (in my idiot youth) fixed an accounting error in an insurance quoting program without first taking it up the chain of accountants, and even though my fix was correct, the drama it invoked brought down an unholy shitstorm of blame and recrimination. Do not fix anything unless someone tells you to, and document, document, document, document. The reporting and documentation requirements of this kind of work are off the charts. If you change anything in existing code (if you add a COMMENT) you will need to document it, and get it approved by someone on the math side.
In short: be honest, document your work extensively, and run all money-related changes by the accounting team until you understand what they want, and they understand what you're going to do.