[GNC] Budget report fails in multiple currencies

[Fred Bone]

On 07 January 2023 at 10:21, ml enquirer said:

> Just to add that I was wrong that this could be solved by making budgets
> with a single period. I *think* this problem arises for any multi-currency
> sub-accounts and becomes visible when you have a book which has been
> running for many years. In that circumstance, the 'actual' column is only
> incrementally affected by spends during the actual reporting period (which
> are small compared to the cumulative spending over the years).
> 
> Imagine a case where there is *no* spend on any sub-account in a reporting
> period, but the years prior total 1,000,000 EUR and 1,000,000 GBP. Imagine
> the exchange rate is 1 EUR to GBP at the start of this new 'empty' period
> and 1 EUR to 1.01 GBP at the end. 1) The computed total at the start will
> be 1,000,000 EUR + 1,000,000 GBP = 2,000,000 GBP. 2) The computed total at
> the end will be 1,000,000 EUR + 1,000,000 GBP = 2,010,000 GBP and the
> reported "Actual" spend will be 10,000 GBP despite nothing having been
> spent.

The change in value is indeed 10,000. What's the problem?