Priority date

Almost every question in patent law is really a question about a date: was this public before yours, or after?

In simple words

The earliest date to which a claim is entitled - usually the date of your first filing for that subject matter. Prior art is judged against it.

Think of it as

A line drawn across time. Everything public before the line can be used against you; everything after it usually cannot.

Why it matters

Priority is claim-by-claim, not application-by-application. Different claims in the same patent can have different priority dates depending on which filing first disclosed them.

A simple example

You file a provisional in March, a complete specification the following March, and a competitor publishes in September in between. For claims fully supported by the provisional, the competitor's publication is too late. For claims supported only by the later filing, it is prior art.

Common mistake

Believing the priority date belongs to the whole application. It belongs to subject matter.

Where it comes from

Patents Act 1970 - sections 11, 9 and 135; Paris Convention priority