The date on the motor can be as much as 90 days prior to the build date of the car. My motor is dated 1/29/70 with a build date on the car of 2/13/70. I did the same thing with my car everything is date coded properly.
And some engines have build dates AFTER the Scheduled Production Date (SPD) of the car. One of my cars the engine is 1 day before and I know of another that is ~10 days after the SPD.
Nothing on these cars is absolute. Production snafu's and assembled unit part delays can account for some weird date coding.