When an international application claims the right of priority of the date of filing of an earlier application, the priority date (i.e. the filing date of the earlier application) will be used to calculate certain time limits.
The priority claim must refer to an earlier application. Since the minimum time limit mentioned under Rule 80 is a day, The day of filing of the earlier application not being included in the priority period (Art. 8(2) PCT in conjunction with Article 4C(2) of the Paris Convention and Rule 2.4(a) PCT), the priority period starts on the day following the date of filing of the earlier application. Thus, an "earlier" application is to be understood as an application that has been filed at least a day before the application claiming priority. This is in line with Rule 2.4 and also with Article 4C(2) of the Paris Convention to which Article 8 refers.
Furthermore, the priority date becomes the effective date for the purposes of international examination, i.e. the written opinion (of either the ISA or the IPEA) and the international preliminary examination report. The relevant date for the purposes of the international search is always the international filing date.
See ISPE Guidelines 6.02.