 About 80 percent of those subdivisions were built since 1983.In late 1989, the United States Postal Service approved the North Potomac name for a region surrounded by the Montgomery County communities of Darnestown, Travilah, Gaithersburg and Rockville.ZIP Codes were not changed, so most of North Potomac uses the 20878 code used by Gaithersburg and part of Darnestown.