This comes up at the very first settlement: the statement shows one figure, the board at the exchange office showed another. The reason is simple — the conversion runs on the Czech National Bank’s official rate for the date of the transaction itself, never on an exchange-office rate or the payment date.
What the ČNB publishes
The national bank publishes an official exchange-rate list on working days. The publication relates to a specific day, and that is the rate used for conversion. There is no “morning” and “evening” value on the list — one day, one number.
Source: ČNB — Kurzy devizového trhu — “Kurzy devizového trhu jsou vyhlašovány pro běžně obchodované měny, a to každý pracovní den kolem 14.30.” The list for any given day can be opened there directly; nothing written here has to be taken on trust.
From which follows the main rule: if the transaction fell on a weekend, a holiday or any other day with no publication, the last preceding available rate is used — always the preceding one, never the next rate or a period average.
The transaction date, not the money date
You fuel up in Poland on the 3rd, the card is debited on the 5th, you invoice on the 20th — the conversion uses the rate for the 3rd. The date on the document is the date of the transaction.
Why it is noticeable
Loads and settlements are in euro, while costs arrive in the currencies of the route: złoty, forint, koruna. Every refuelling and every toll is converted on its own date. On a single receipt the difference between “the rate on the transaction date” and “the rate on the payment date” is invisible. Across a hundred receipts in a month it becomes a line you can see with the naked eye.
That is exactly why you cannot do the conversion by hand at the end of the month at one “average” rate: the result comes out approximately right, and then you have to explain the discrepancy.
And because the costs being converted are the ones that reduce your tax base, the rate is part of the tax record rather than accounting cosmetics — which costs those are is set out in Which costs actually reduce the tax base.
How it looks in the portal
The rate is pulled in for the date of each transaction automatically, along with the cost import, and stored next to the amount. In the settlement you see the original currency, the rate and the result in euro — so you can check any line without leaving the document and without opening the national bank’s website.
What has to be together before the annual close is covered in Your first annual return.
