GHE was an era of time on Bajor. In 351 GHE, the aqueduct at H'erat was built. (TNG: "Birthright, Part I" ) The date was mentioned in a Okudagram seen on Picard's monitor in his ready room aboard the USS Enterprise-D. A clear view of the graphic was on page 181 in the Star Trek: The Next Generation - The Continuing Mission.