Article 4

1.   A European patent with unitary effect shall take effect in the participating Member States on the date of publication by the EPO of the mention of the grant of the European patent in the European Patent Bulletin.
2.   The participating Member States shall take the necessary measures to ensure that, where the unitary effect of a European patent has been registered and extends to their territory, that European patent is deemed not to have taken effect as a national patent in their territory on the date of publication of the mention of the grant in the European Patent Bulletin.

UPC Case Law

IPPT20260116, UPC CFI LD Paris, IMC Creations v Mul-T-Lock 
 A unitary patent takes effect retroactively, (Articles 64(1) , 68 EPC , Article 4(1) Unitary patent regulation) as of the date of the European patent became a unitary patent, pursuant to the combined provisions of Articles 64(1) and 68 of the EPC and Article 4(1) of the Unitary patent regulation.  Furthermore, the patent as granted, so long as it has not been judicially invalidated, is deemed valid from the date of its grant.