A software program which implements predictive phenology modeling is a valuable tool for growers and scientists. Such a program was created in the late 1980's by the creators of general phenology modeling as proof of their techniques. However, this first program could not continue to meet the needs of the field which created it and needed to be expanded or replaced. This paper presents an overview of the aims of phenology modeling and the legacy program which demonstrated it. The paper then discusses the process of reverse-engineering the legacy program and specifying the functionality for its successor. The result is a program which implements all the functions of the legacy code while providing a modern platform that is maintainable, portable, and open.