In order to obtain a patent, the invention must be new or "novel." If the invention is described in a printed publication or patent anywhere in the world, or, is in public use or on sale in the US for more than one year before the filing date of a patent application, then that invention is not novel. Thus, an inventor has a one-year grace period from the time any of the above activities are conducted to file a US patent application.
The grace period described above does not exist in most foreign countries. Most foreign countries are what is called "absolute novelty" countries. Absolute novelty means that these countries do not allow any grace period from the time an inventor describes the invention in a printed publication or patent anywhere in the world, publicly uses or sells the invention or discloses the invention to other, to the time the patent application is filed. Failure to file an application before any such activity occurs results in a loss of patent rights in these countries.