Each Kitty has a currency that is specified on Kitty creation. We call it the home currency. Expenses and debts are all displayed in that currency.
If you want to work with multiple currencies in a Kitty (for example, because you're on an international trip) then you need to upgrade your Kitty first. Afterwards, you can add expenses in any currency you want and Kittysplit will convert the amount back to your home currency. All debts will be displayed in your home currency.
We fetch the exchange rates from swop.cx, a simple and lightweight internet service for current and historical foreign exchange (forex) rates. To make it as fair as possible, we fetch the exchange rate for the given date of the expense, which we feel is rather smart 🤓. Editing an expense will only change the rate if the date changes too. We keep the most current rates for all currencies and a refresh every 12 hours.
Please note that you can change the home currency of a Kitty only as long as all expenses are in the same currency. Once you have a mix of currencies you can no longer change the home currency. Sorry!