Passage: With Switzerland joining the Schengen Treaty in March 2009, the air side was rearranged to include a Schengen and non-Schengen zone. As border control is staffed by both Swiss and French border officers, passengers departing to or arriving from non-Schengen countries may receive either a Swiss or French passport stamp, depending on which officer they happen to approach.

Question: when did the Switzerland join the schengen treaty?
March 2009