BT (also B.T. or b.t.): Before Tranquillity. Any date before 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 Tranquility Calendar that became standard in Solsys during the Federation Age.