I don't have an iPhone so I can't try it but I read on the website "As you can see, there is no "exchange rate" here. I decided to keep as much data separated

I don't have an iPhone so I can't try it but I read on the website "As you can see, there is no "exchange rate" here. I decided to keep as much data separated as possible since a user may want to change their currencies around." Not storing the exchange rate if the rate changes a lot don't you risk having a value that might not be correct? For example if my main currency is EUR and I store a transaction of 5 US $ with an exchange rate of 1€ = 1.3$ then if one week later it becomes 1€ = 2$.