The assembly date could be within three months typically of your build date. What you want to check is the assembly date of the block which is stamped on the front driver's side below the cylinder head.
The casting date could be anytime before your car, even up to a year (not typical)
Also if you're looking for blocks always check the VIN. In your case it should start with a "1B" if it does then you know it even came from the same assembly plant as your car, then obiviously the rest of the VIN will indicate whether or not it was made before or after your car.
