On 21 December, the European Commission adopted rules relating to the EU Digital COVID Certificate, establishing a binding acceptance period of 9 months (precisely 270 days) of vaccination certificates, ..., for the purposes of intra-EU travel
https://ec.europa.eu/info/live-work-travel-eu/coronavirus-response/safe-covid-19-vaccines-europeans/eu-digital-covid-certificate_en
Man beachte, welche Grenzen hier wohl gemeint sind.