I believe the date moved to the 27th (according to the Playtest homepage).
EDIT: Quote from the Playtest page
Preorder Update: Paizo.com preorders for Pathfinder Playtest products will begin March 27. We are delaying the start to provide better shipping options, including picking up at Gen Con 2018! The preorder period will still end at 11:59 PM Pacific on May 1.