You really don't need a date code to tell that the tire is not the OEM spare. Just the alpha-numeric tire size (H78-15) is enough to know that it's well over 6 years old.
When new, that was a pretty good tire, by observation. Kelly was a subsidiary of Goodyear, so that's at least a decent indication of its quality. (That does NOT mean it has the same exact rubber and such in it that Goodyear used on their own-brand tires, though!) BUT that was then, when the tire was new.
You can keep it as a conversation piece and a general information guide for what tires the car used to have, but that's all, at this point in time.
With age, even if the tread is deep, the inner tire cords will "dry rot", unseen, and the tire will fail at some point in time. You might swerve to miss an animal in the road, or make an emergency stop, and that would be enough stress to make the tire come apart internally, lose air, and fail.
Get a used wheel, paint it nice, and put a new tire on it. Can even be "cheap, black (with whitewall), and round", if you desire. There should be a jack decal on the inner side of the deck lid to illustrate how the jack/spare tire are stored and the attachments to do so.
What are the other tires on the car?
CBODY67
