This paper explains the design and computer implementation of a comprehensive dynamic air traffic system simulation program known as ComDATSS, currently under development at the University of Minnesota. ComDATSS simulates all essential components of an air traffic management system and their dynamic interactions. It provides a numerical testing environment for evaluating advanced ATM system designs and automation tools. Compared with existing ATM system simulation programs, ComDATSS employs an "as-it-is" physical modeling approach, generates four-dimensional aircraft trajectories, and allows for sufficient randomness in the simulations. In addition, the computer implementation of ComDATSS is implemented as a modular structure that can easily be modified and maintained.