I build a multi-currency personal-finance app. This is one of the first bugs I had to design my way out of — and it's one almost every multi-currency app I've tried still has. The symptom You spent 200 zł on groceries in March. Your base currency is euros. In March the app showed that as, say, €46. You open the app in July and that same March transaction now reads €43 — even though nothing about March changed. Multiply that across every foreign transaction and your "total s...