Add Shared Expenses in a Foreign Currency, Converted Once and Kept

Every nest in Eider has one base currency, chosen when the nest is created. Turn on foreign currency in the nest's settings and an expense can be entered in a different one. A currency pill sits beside the amount, and the keypad follows whatever you pick.

Eider converts the amount while it saves the expense. Balances stay in the nest's base currency, so a 50 euro dinner in a dollar nest moves the balance in dollars, by the converted figure. Stored beside that figure are the original amount, the rate that produced the total, and the date and service that rate came from. The expense detail shows them back to you under Original and Rate.

The conversion runs once and is never repeated. Open that dinner next spring and it reads as it read the day it was added, whatever the euro has done since.

1. 1
   You enter the amountIn the currency you paid in. The pill beside the amount is where you pick it.
2. 2
   Eider converts itOnce, as the expense saves, at the rate for that expense's date.
3. 3
   The rate stays with the expenseNothing re-converts it later. The figure in the balance is the one that stands.

An app holding a foreign expense can convert it every time somebody looks, or convert it once and keep the answer. The first is less work to build. It also means every balance in the nest depends on a rate fetched at the moment of reading, and moves whenever that rate moves.

Eider converts once, at the rate for the expense's own date, and keeps the result. The foreign amount and the rate ride along as a record of what happened, not as figures anything computes from. Nothing downstream reads them: the balances, the per-pair totals and the settle-up figures all work from the converted amount.

That is what lets a settled period stay settled. Closing a period fixes the expenses in it, so the amounts behind the balances people squared up on can't move afterward. A rate that re-converted on every read would move them anyway, quietly, months later, inside a period nobody is allowed to edit.

The rate's own date isn't always the expense's date. Rate providers publish on business days, so an expense dated a Sunday pins the preceding Friday's rate. The Rate caption on the expense detail names that date and the service it came from, rather than implying the rate belongs to the day the money was spent.

Foreign currency is off in every nest until somebody turns it on, trip nests included. The setting lives in the nest's settings under Currency settings, it flips both ways, and turning it back off only removes the entry control. Expenses already added keep their pinned rates and go on showing their originals.

The base currency is the other half of this and behaves differently. It's chosen when the nest is created and there's no way to change it afterward. One currency per nest is what keeps the balances from ever mixing two, which is why it has no edit screen.

There are 26 currencies to enter in, a fixed list rather than every code in circulation. Zero-decimal currencies are in it, which is where the keypad gets its shape: the yen and the Korean won take no decimal point, so the keypad doesn't offer one. Currencies with three decimals, like the Kuwaiti dinar, aren't on the list at all.

While you're typing, a line under the amount says what the expense will cost the nest. At zero it quotes the rate on its own. Once there are digits it shows the converted total and says it's at today's rate. If the rate Eider holds is more than five days old, the line names the day it came from instead.

Balances and settling up are always in the nest's base currency. Recording a payment has no currency pill and no conversion, because a payment is a number two people have already agreed on, in the currency they share.

A repeating expense can't be foreign. The pill disappears as soon as Repeats is switched on, and that's a refusal rather than a gap. A repeating rule stores an amount and not an amount plus a rate, so 3,000 yen left on a monthly schedule would be filed as 3,000 of the nest's own currency and charged that way every month after.

A backdated expense won't match what the sheet showed you: the line quotes today's rate, while the saved expense uses the rate Eider holds for the expense's own date. How far apart the two land is whatever the rate did in between. For three of the 26 there's no older rate to be had at all, so Eider pins the nearest date it holds and names it.

All of it is part of the free product. There's no limit on how many foreign expenses a nest can hold and nothing to switch on past the setting itself.

An expense carries the rate it was converted at. A balance settled last spring is still settled at last spring's rate.