The trial for 23-year-old Andrew Gallo of San Gabriel was scheduled for Monday in Orange County but that's been pushed back to September.
The change was granted Friday after Gallo's lawyer said she needed more time to prepare an expert witness.
Gallo's minivan struck a car at a Fullerton intersection in April 2009. The 22-year-old Adenhart was out with friends after pitching six scoreless innings against the Oakland Athletics in nearby Anaheim.
Authorities say Gallo's blood-alcohol level was nearly three times the legal limit.
He's pleaded not guilty to three counts of second-degree murder.