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What is a priority date for a patent application?  

The term “priority date” can mean any of several different things, depending on the context. The “priority date” of a patent application is the date which controls what prior art affects its patentability, whether a statutory bar applies, etc. For many patent applications the priority date is the date on which the patent application was filed with the patent office. In some circumstances a patent application will enjoy an priority date earlier than its own filing date. For example, a continuation or divisional application will enjoy the filing date of the application of which it is a continuation or divisional. Another example is the application which claims priority under the Paris Convention from a counterpart application filed less than one year earlier in another country; such an application has a priority date equal to that of the earlier application. Yet another example is the application which claims priority from a provisional application. Finally, an application may enjoy a priority date by virtue of being a national-phase application arising out of a PCT patent application in which the United States was designated. There are other circumstances in which a patent application will have a priority date differing from its filing date.