What is the ‘effective filing date’ for a claimed invention?

The ‘effective filing date’ for a claimed invention is defined in 35 U.S.C. 100(i)(1) as follows:

The term “effective filing date” for a claimed invention in a patent or application for patent means—
(A) if subparagraph (B) does not apply, the actual filing date of the patent or the application for the patent containing a claim to the invention; or
(B) the filing date of the earliest application for which the patent or application is entitled, as to such invention, to a right of priority under section 119, 365(a), or 365(b) or to the benefit of an earlier filing date under section 120, 121, 365(c), or 386(c).

In simpler terms, the effective filing date is either the actual filing date of the patent application or, if applicable, the filing date of the earliest related application from which priority or benefit is claimed for the invention.

For reissue applications, 35 U.S.C. 100(i)(2) states:

The effective filing date for a claimed invention in an application for reissue or reissued patent shall be determined by deeming the claim to the invention to have been contained in the patent for which reissue was sought.

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