Also, sometimes currencies require timestamps. You don't represent money as (<currency>, <decimal amount>), you may want to store it as (<currency>, <decimal a

Also, sometimes currencies require timestamps. You don't represent money as (<currency>, <decimal amount>), you may want to store it as (<currency>, <decimal amount>, <timestamp>) to be able to apply the correct exchange rate post-facto, and not have to deduce from the transaction history. It also helps with cross-checks/consistency verification. Also, preserve the original timezone in the timestamp. It can save many headaches down the road.