37 CFR §1.109

1. (a)The effective filing date for a claimed invention in a patent or application for patent, other than in a reissue application or reissued patent, is the earliest of:
   1. (1)The actual filing date of the patent or the application for the patent containing a claim to the invention; or
   2. (2)The filing date of the earliest application for which the patent or application is entitled, as to such invention, to a right of priority or the benefit of an earlier filing date under 35 U.S.C. 119, 120, 121, 365, or 386.
2. (b)The effective filing date for a claimed invention in a reissue application or a reissued patent is determined by deeming the claim to the invention to have been contained in the patent for which reissue was sought.