AT (also A.T. or a.t.): After Tranquillity. Any date after the first landing of Terragens on another celestial body, when the Apollo 11 mission achieved touchdown on the Old Earth's Moon. This was July 20, 1969 c.e. or AD in the Gregorian calendar, and Armstrong Day, 0 AT in the later Tranquillity Calendar that became standard in Solsys during the First Federation era.