Where only a single patent application is involved, the priority date would obviously be the filing date of the sole application. If an applicant has filed a number of related patent applications, the priority date would be the filing date of the earliest patent filing that first disclosed the invention.
(4) A provisional application is not entitled to the right of priority under 35 U.S.C. 119, 365(a), or 386(a) or § 1.55, or to the benefit of an earlier filing date under 35 U.S.C. 120, 121, 365(c), or 386(c) or § 1.78 of any other application. No claim for priority under 35 U.S.C.
Priority date refers to the earliest filing date in a family of patent applications. If the earliest-filed patent application for a particular invention was a provisional application, then the filing date of the provisional is your priority date.
A priority claim is a helpful, and often critical, way to link a later-filed patent application to an earlier-filed patent application. Known as a priority application, the earlier-filed application must generally have common subject matter and common inventorship in order for a priority claim to be made.