A new mixed-time representation for STN-based scheduling models is proposed: the time grid is fixed, but processing times are allowed to be variable and span an unknown number of time periods. The proposed representation handles batch and continuous processes, and it accounts linearly for holding, backlog, and utility costs. It handles release and due dates at no additional computational cost, and it accounts for variable processing times. Computational improvements for the solution of the proposed mixed-integer programming formulation are also developed.