Pick the date your certificate was issued. It is printed on the certificate and shown on the register entry.
Worked example
A certificate lodged on 15 March 2019 is valid until 15 March 2029: issue date plus 10 years GOV.UK. On 2 August 2026 that certificate has roughly two years and seven months to run, so a seller could market on it twice over before renewal is even a question.
Do not trust your memory over the register
The register entry shows the authoritative expiry date for any property, free GOV.UK register. If the date this tool gives you disagrees with the register, the register wins; you may be remembering the survey date rather than the lodgement date.
Expiring soon?
There is no automatic duty at expiry: owner-occupiers can let it lapse without consequence. Sellers and landlords should time renewal deliberately, because assessments now run under RdSAP 10 and can score lower without documentation Elmhurst RdSAP 10. The full decision table is on should I renew my EPC now, validity nuances on how long an EPC lasts, and the changes your next certificate will live through on the reform tracker.