Priority date is a phrase we may or may not use very often, but it is the earliest filing date pertinent to any given invention, within the international context. It is a key point of Industrial property law and recognition of any said date is a key aspect of the Convention de Paris pour la protection de la propriété industrielle.
It pertains to any patent or any document legally equal to a patent, such as the French Certificat d'addition. In the case of these latter, or in the case of any international analogue thereto, the “priority date” is not the filing date of the “parent patent”, but that of the specific innovation as articulated in the addition.
Across our data, accessed by way of either scanned antique documents or type-entered data on any national of international database, we find great variation in terms of how fastidiously this date is entered. We often have to infer it in the process of our finding patents filed internationally. This fastidiousness varies from nation to nation, as do many aspects of administrative culture. In the case of patents filed in Hungary, there is a fair chance that an initial non-Hungarian date will be explicated as such, with the “Elsőbbsége”(“Priority”), followed by the appropriate date, being displayed on the patent document original. Hungary is an interesting case also in that, even with said data lacking on the original, the appropriate date will often be found entered within the modern digital-type-entered information.
In terms of nice display on antique original patent documents the Austrian “Priorität vom” is among the variations on this nicety. In the case of other nations, related phraseology, per se, is not used, though a reference to a predecessor patent filed in some other nation will offer the same data. Again this is not a matter of per se “additions” relative to “parent patents”, all filed within one nation, but a matter of exactly analogous innovations filed across the international context. British and French patents documents often comport well with these niceties, making reference to the Convention de Paris pour la protection de la propriété industrielle.
Regarding Espacenet, one consistent glitch we have noticed pertains to Spanish patents. A column of results based on an inventor's name-based search will show alleged priority date data. Said data, in the case of Spanish patents, and only Spanish patents, is NOT the actual priority date, as defined above, but the filing of the Spanish patent itself.