Selected by NASA in January 1990, Halsell became an astronaut in July 1991. A five flight veteran, Halsell has logged over 1,250 hours in space. He was the pilot on STS-65 (July 8-23, 1994) and STS-74 (November 12-20, 1995), and commanded STS-83 (Apr 4-8, 1997), STS-94 (July 1-17, 1997) and STS-101 (May 19-29, 2000). From February-August 1998, he served as NASA Director of Operations at the Yuri Gagarin Cosmonaut Training Center, Star City, Russia. Halsell also served as Manager, Shuttle Launch Integration, Kennedy Space Center, Florida, from July 2000-January 2003. Halsell was Space Shuttle Program manager for launch integration at the Kennedy Space Center from 2000-2002, responsible for giving the "go for launch" on 13 Shuttle missions.  After the Columbia accident, he led the NASA Return to Flight Planning Team, responsible for converting the recommendations of the accident investigation board into Shuttle Program actions that resulted in resumption of missions in 2005. Following the Columbia accident, Halsell led NASA’s Space Shuttle Return-to-Flight Planning Team. He then served as the Assistant Director for Aircraft Operations, Flight Crew Operations Directorate. Halsell retired from NASA in November 2006 to accept a position with  ATK Launch Systems where he served as Vice President, Space Exploration Systems, and was the ATK site manager at Marshall Space Flight Center, Huntsville, AL.
Question: How many months after being selected by NASA did Halsell become an astronaut?
Answer: 18