This paper describes CONFIG, an implemented hybrid modeling and simulation approach, within a discrete event system framework, and its application to control software validation. CONFIG system models are made up of connected devices and activities, and device models are made up of mode transition models, with behavior models for each mode. Behavior models can be discrete or continuous, and characterize how changes in discrete inputs produce changes in outputs and mode transitions. In CONFIG, two operators, Integrate and Apply-When, are used to compute states or time advances that depend on continuous changes. The Apply-When operator calls external algebraic functions to determine the time advance for a rate-dependent event. The Integrate operator uses a discrete-time approach, providing periodic updates of variables. CONFIG simulations were used to validate advanced control software for gas transfer between chambers during a 90-day manned test of technology for Lunar-Mars life support.