This limit on the year digits is the cause of the Y2K problem. Data compression is a possible technique to obtain the intermediate format. One method or algorithm is based on the fact that the Gregorian Calendar has a 400-year period. For a fixed day of the week, day, and month, there is a unique century in any given 400-year period. Suppose the format were C42599.
Many sources supply the association.
For example, the CRC Standard Mathematical Tables and Formulae give day of the week for any given day. Intervals of 400 years each have a unique character so representations are unique over many more than the 400-year cycle in the first method. In the same way that hexadecimal numbers can denote months, larger number bases can be used to create more compression. Obviously, there are many data compression formats. The first example meets the criteria of readability, detectable by software, with low complexity.