In February, the auditor of a small Zurich trading company asks for one thing: the supplier invoice behind a large December booking, as it looked on the day it was booked. The bookkeeper finds a PDF. It opens fine. But the auditor's next question is the one that takes longer to answer. How does anyone know this is the file that was on the system in December, and not a copy that was tidied up in January?
That question is the whole subject of a year-end document audit trail. Before the books close, it helps to know which records you will be asked to show and what each kind of evidence can actually prove.
What the Swiss rules ask of your records
The Code of Obligations starts with the voucher. Under Article 957a of the Code of Obligations, bookkeeping follows proper accounting principles, and those include proof of the underlying voucher for each booking and the possibility of checking it afterwards. A voucher can be on paper or in electronic form. The annual report has its own timetable: Article 958 gives six months after the end of the financial year to prepare it and put it to the body that approves it, and it has to be signed.
Then comes retention. Article 958f sets ten years, counted from the end of the financial year, for the books, the vouchers, the annual report and the audit report. The annual report and the audit report are kept in writing and signed. Books and vouchers may be kept electronically, as long as they still match the underlying transactions and can be made readable at any time.
The Federal Council's Ordinance on the Keeping and Retention of Accounting Records (GeBüV) fills in the detail for electronic records. Article 3 says records must be kept so that they cannot be changed without that being noticeable. Article 9 goes further for storage that can be altered: the integrity of what is stored has to be secured by a technical method, and the time of storage has to be provable in a way that cannot be falsified. The ordinance gives a digital signature as an example of the first and a time stamp as an example of the second. It also asks for the procedures to be documented and for the related logs to be kept.
Four things an auditor may ask you to show
It helps to separate them, because one piece of evidence rarely covers all four.
That the file existed at a given time
A file's own "modified" date is set by whoever holds the file, and anyone can change it. A qualified electronic time stamp is different. Under Article 41 of eIDAS, it carries a presumption that the date and time it shows are accurate. Whoever disputes it has to bring the evidence.
That it has not changed since
The same Article 41 presumption covers the integrity of the data the time stamp is bound to. In practice a hash of the exact file is bound to the time, so a changed file no longer matches. This is the property GeBüV Article 3 is describing in general terms.
Who signed
A signature on the document shows who approved it. The annual report is the obvious case, since Article 958 requires it to be signed. Whether a scanned signature, an advanced one or a qualified one is enough depends on the form the rule asks for, and that is a question for your accountant, not for a time stamp.
Who the person was
This is the gap people forget. A time stamp says nothing about who put the file there. If a counterparty or an authority asks who made or approved a record, the answer needs an identity check behind the name. Swiss Trust Layer's identity check ties a sealed record to a person verified against a passport, so the name on the record has something behind it.
What sealing does, and what it does not
Sealing a file on Swiss Trust Layer records a hash of the exact file together with a qualified time stamp. Later, anyone can check the file against the seal, and the check either matches or it does not. That answers the first two questions above.
It does not answer whether the invoice was correct, whether the booking was right, or whether your accounts are complete. It is not bookkeeping software, and it does not make your books compliant. GeBüV also asks for documented procedures and retained logs, and a seal does not write those for you. Whether your whole setup meets the ordinance is something for your accountant or auditor to judge.
A practical order for the last weeks of the year
Start with the records that are hardest to reconstruct later: bank statements, the invoices behind the largest bookings, stock count sheets, signed contracts, board or owner approvals. Seal them when the set is final, and keep the seal certificates next to the files. If you migrate records to a new system, log the transfer as Article 10 of the ordinance asks, and check that the files still match their seals afterwards.
Then note, per record, which of the four questions it can answer. Gaps show up quickly. A supplier contract may have a time stamp and no identity behind it. A signed annual report may have a signature and no proof of when the final version existed.
Before the auditor asks
The auditor in our opening example does not need a story about December. They need a file and a check that agrees with it. If you want that for your own year-end set, start sealing your records, or read how the process works first.
