I assume this is because they are using the new Kappa architecture? Any other reasons for production delay? It seems like it normally does not take 2 years to go from finished production design to begin of production.
Likely we are looking at about another year and a half most likely - production will likely start in late summer, with dealer deliveries starting in early fall. Expect GM to use the first production vehicles for early testers to employees to work out any final bugs/problems. Dealers will probably start taking pre-orders in the spring if everything holds to their current plans.