On November 21, 2016, the U.S. Customs and Border Protection updated the list of countries (found here) that belong to the “Six-Month Club”.
The Six-Month Club refers to citizens of specified countries on this list who, as nonimmigrant visitors, only need to have a passport valid for their intended period of stay in the United States. These citizens are exempted from the general rule that visitors traveling to the United States must have passports that are valid for at least six months beyond their intended period of stay.
This list is periodically adjusted by the USCBP.