Filed application for patent filed in the United States may claim the benefit of, or priority to, a prior application filed in the United States. What is a patent priority date?Priority date refers to the earliest filing date in a family of patent applications. The priority date determines whether another patent filing or publicly available document qualifies as prior art against one's patent application. The priority date of a patent relates to the earliest filing date from within a family of patent applications. A priority claim is made to an earlierfiled patent application. For example, an Applicant can claim priority in a later-filed patent application. In Espacenet you can find the priority date in the Bibliographic data section right after the Priority number(s) in the format yyyymmdd. Priority date (The priority date is an assumption and is not a legal conclusion.