There are quite a few standards - "closing the books" on some period (monthly, daily, hourly, depending on volume/regulatory requirements) is usually the way.

There are quite a few standards - "closing the books" on some period (monthly, daily, hourly, depending on volume/regulatory requirements) is usually the way. There's also a good, sadly now out of print, book named "Analysis Patterns" by Martin Fowler which has a large section about account representation strategies from an object modelling perspective which also translates reasonably well to functional programming. It's annoying it's out of print, since it contains timeless information rather than the fad-du-jour!