The date on which the earlier application was filed must fall within the priority period of 12 months preceding the international filing date. However, if the international filing date lies after, but within two months of, that period's expiry, a priority claim will not be considered void for the purposes of the international phase of the PCT procedure, irrespective of whether restoration of the right of priority is requested (see GL/PCT‑EPO A‑VI, 1.5).
The priority period starts on the day following the date of filing of the earlier application. The RO/EP will thus not accept a priority claim relating to an application having the same date of filing as the international application and will inform the applicant that the claim will be disregarded unless the priority date can be corrected (PCT/RO/110; see GL/PCT‑EPO A‑VI, 1.4.1).
If the last day of the priority period falls on a day on which the EPO is not open for the receipt of documents or on which no mail is delivered due to an official holiday or other circumstance described in Rule 80.5, it expires on the next subsequent day on which none of these circumstances exists. However, the priority period may not be extended under Rule 82 or Rule 82quater in the event of irregularities in the mail service.
Art. 4C Paris Convention