Agreed with both of you, and using an exchange rate at a specific point in time actually could make deferring that calculation even more of a sound idea, depend

Agreed with both of you, and using an exchange rate at a specific point in time actually could make deferring that calculation even more of a sound idea, depending on the exact semantics of what you’re computing. At larger scales even exchange rates are a leaky abstraction over a forex market, so there you go. No solution will work everywhere. But if you’re entering the prices of last week’s coffee and hotel room into a web app to see how much you’ll be reimbursed, you don’t want to use a new exchange rate every time a form input changes. Source: worked on multi-currency expense reporting software where everything is done in JavaScript floats.