If you stuck a 350Z and a new Camaro in a blender the result would look like the new GTR. What date the concept came out is pretty irrelevant. Now if you were trying to establish that the new Camaro is a styling rip off of the GTR then the dates would be relevant. For a "this car looks like these two cars combine" statement it doesn't really matter when any of them came out.
For example: The new Beetle looks like a cross between an orignal Beetle and first year Dodge Neon.
1989 240SX Fastback. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1995 300ZX Twin Turbo Warning: Objects in mirror aren't as fast as they thought they were.