In the U.S., a patent is granted to the "first to invent" an invention. The date of invention, is the date the invention was completed or "made." An invention is made when an invention that has been conceived (thought of) by its inventor(s) is reduced to practice by him/her/they or someone else on behalf of those inventor(s). An invention is reduced to practice either by building and successfully testing it (at an appropriate location) or by filing a patent application that adequately discloses it. If the issue arises during patent prosecution, the inventor(s) may be required to prove (by submitting uncorroborated evidence) that both acts (conception and reduction to practice) occurred when he/she/they said they did and that he/she/they were reasonably diligent in reducing the invention to practice after conception occurred. If the validity of the issued patent is ever challenged, the inventor(s) may be required to prove when these acts were accomplished with evidence corroborated by other than his/her/their own testimony, e.g., by the testimony of one or more witnesses.
Under U.S. law, the date of invention is the earlier of the date that the conceiver(s) of the invention [the inventor(s)] commenced being diligent in reducing the invention to practice in the U.S. (assuming that the inventor(s) were reasonably diligent from that date until reduction to practice actually occurred) OR the date that the first patent application was filed that adequately disclosed the invention [35 U.S.C. 102(g)]. The first patent application can be a non-U.S. application (35 U.S.C. 119). U.S. law encourages early filing of U.S. patent applications by denying an inventor the right to rely on a date of invention more than 12 months prior to the date a U.S. patent application or an international (PCT) patent application designating the U.S. is filed on the invention [35 U.S.C. 102(b)].