This question asks if a priority claim under the Paris Convention can be made after the 12-month deadline, and the accepted answer is 'no'. However, I wanted to ask again if this is really the case. Specifically:
(i) Does the Paris Convention or PCT allow for an extension (e.g. due to 'unintentional' error)?
(ii) Do individual countries allow for an extension?
It appears that the PCT in fact mandates ('shall') a 2-month extension for unintentional delay.
PCT 26 bis.3(a)
Where the international application has an international filing date which is later than the date on which the priority period expired but within the period of two months from that date, the receiving Office shall, on the request of the applicant, and subject to paragraphs (b) to (g) of this Rule, restore the right of priority if the Office finds that a criterion applied by it (“criterion for restoration”) is satisfied, namely, that the failure to file the international application within the priority period: (i) occurred in spite of due care required by the circumstances having been taken; or (ii) was unintentional.
See MPEP 213 for the US implementation.
35 USC 119
The Director may prescribe regulations, including the requirement for payment of the fee specified in section 41(a)(7) , pursuant to which the 12-month period set forth in this subsection may be extended by an additional 2 months if the delay in filing the application in this country within the 12-month period was unintentional.
37 CFR 1.55(c)
Delayed filing of subsequent application. If the subsequent application has a filing date which is after the expiration of the period set forth in paragraph (b)(1) of this section, but within two months from the expiration of the period set forth in paragraph (b)(1) of this section, the right of priority in the subsequent application may be restored under PCT Rule 26bis.3 for an international application, or upon petition pursuant to this paragraph, if the delay in filing the subsequent application within the period set forth in paragraph (b)(1) of this section was unintentional. ...