The question arrives in writing, often years after the year under review: a request for information asking you to justify, day by day, where the work was carried out. The time allowed to answer is short — in Belgium, one month as a rule — and it is not extended on the grounds that the evidence is hard to find. Everything that follows is therefore prepared before the letter, not after.
What the tax authority asks for
The format varies, the substance is stable:
- an annual count: days worked in Luxembourg, days worked elsewhere, holidays, sick leave, public holidays — the total has to land on the working days of the year;
- the records that support that count, day by day for the days in question;
- often, an employer's certificate summarising the days worked outside Luxembourg.
The authority does not ask you to prove every Monday of the year; it holds the count up against the records and looks for the places where the two do not match.
The three classic inconsistencies
The count that changes from one document to the next. 31 days outside Luxembourg in the tax return, 36 in the employer's certificate, 33 in the table submitted in reply: each document is plausible on its own, the set is indefensible. The count has to exist in a single copy, kept up to date, from which everything else derives.
The record that contradicts the status. A card payment at midday near your home on a day declared "in Luxembourg"; a Luxembourg badge swipe on a day declared as remote work. A single documented contradiction is enough to cast doubt on the whole register — it is the mirror image of the body-of-evidence argument, and it plays against you.
The total that does not add up. A year has its working days; leave, public holidays, sick days and days worked have to fill them exactly. A count that leaves orphan days invites the authority to classify them itself — rarely in the taxpayer's favour.
The file that closes the discussion
It comes down to three items, consistent with one another:
- the annual register, one status per day, closed and dated;
- the supporting records attached to the days, not in bulk — a receipt with no date and no attachment proves nothing;
- the employer's documents — contract, certificate, badge records — that tell the same year as the register.
A file of that kind is built in ten seconds a day across the year, or in several weeks of reconstruction under a one-month deadline. The final content is the same; the credibility is not: a record contemporary with the facts is always worth more than a table filled in the day before the reply.
And if the file has gaps
You flag them, you do not paper over them. An honest register with a few undocumented days is still a body of evidence; a smoothed-over register that cracks at the first check is not one any more. Where the two authorities keep disagreeing about the same year, the treaties provide for a mutual agreement procedure between States — slow, but made for exactly this.