If you are talking about the pump date code the 68 & up pumps are a Julian date where each day of the year is a consecutive number from 001 to 365. After the first three #'s will be a single number representing the year.
Finally there is another & letter at the end which , I think, is the shift.
So an example would be 24981s decoding the 249th day of 1968
Simple Simon.
1966 & 1967 are done totally different.