The 4 digit date code on tires didn't start until the year 2000. Before that they were a 3 digit code like 338 which could have been the 33rd week of 1998, 1988, or possibly 1978. It is not really clear when manufacturers started using the the 3 digit codes. I would guess the code you are reading is not the date code but some other code stamped into the sidewall. If it is the date code then it may have been stamped wrong and should probably read 3308 since it is 4 digits.
Scott
