What Is a Priority Date?

The priority date is the earliest filing date to which a patent application can claim a right of priority. It is the single most important date in patent law — because it determines which prior art can be cited against your claims and whether your invention is considered novel.

Everything published before your priority date is potential prior art. Everything published after it is not. Moving your priority date one day earlier or later can be the difference between a granted patent and a rejection.

How the Priority Date Is Established

The priority date is established by the first patent application filed for the invention — in any country that is a member of the Paris Convention (which includes virtually every country with a patent system).

Scenario 1: You file a US provisional application on 15 March 2026. Your priority date is 15 March 2026. Any PCT application, national filing, or regional filing you make within 12 months, claiming priority to the provisional, gets the benefit of that date.

Scenario 2: You file a PCT application on 10 March 2027, claiming priority to the US provisional filed on 15 March 2026. The PCT application's effective date for prior art purposes is 15 March 2026 — the priority date — not 10 March 2027.

Scenario 3: You file directly in China on 14 March 2027, claiming Paris Convention priority to the US provisional. The Chinese application's priority date is 15 March 2026. Prior art published between 15 March 2026 and 14 March 2027 cannot be cited against you (for the subject matter disclosed in the provisional).

The 12-Month Priority Window

The Paris Convention grants a 12-month right of priority from the date of the first filing. During this window, the applicant can file in any other member country while claiming the same priority date — as if they had filed on the same day as the original.

This window is absolute. At 12 months and one day, priority is lost. There is no extension, no revival, no exception. This is why article 63 of the iInvent Encyclopedia calls the 12-month deadline "the deadline that catches the most inventors."

Priority Date vs Filing Date

These are different dates, and confusing them causes costly mistakes:

Filing date: The date the specific application was received by the patent office. Every application has its own filing date.

Priority date: The date of the earliest application to which a claim of priority is validly made. Multiple applications can share the same priority date through the priority chain.

The filing date matters for calculating the patent term (20 years from the filing date of the non-provisional or PCT application, not from the priority date). The priority date matters for determining what counts as prior art.

This creates a subtlety: a patent with a priority date of 1 January 2025 and a filing date of 1 December 2025 gets the benefit of the early priority date for prior art purposes but expires 20 years from the later filing date. The priority date protects against prior art; the filing date determines patent term.

Priority and Provisional Applications

A provisional application's primary purpose is to establish a priority date at minimal cost. The provisional itself never becomes a patent — it expires after 12 months. But the priority date it creates lives on through every subsequent application that claims priority to it.

The critical requirement: the provisional must adequately describe the invention. Claims in a later non-provisional or PCT application only get the benefit of the provisional's priority date for subject matter that was actually disclosed in the provisional. New features added in the non-provisional that were not in the provisional get only the non-provisional's later filing date — and are exposed to any prior art that appeared during the gap.

Sources

1. Paris Convention Article 4 — Right of Priority — International treaty establishing the 12-month priority right
2. 35 U.S.C. § 119 — Benefit of Earlier Filing Date — US law governing priority claims from foreign and provisional filings
3. EPC Article 87 — Priority Right — European priority right provisions under the EPC
4. WIPO PCT System — PCT priority provisions for international applications