This paper presents an approach for designing software for dynamical systems simulation. An algorithm is proposed to obtain a schedule for calculating each phase variable of a stiff system of differential equations. The problem is classified as a fixed-priority pre-emptive scheduling of periodic tasks. The Branch-and-Bound algorithm is modified to minimize the defined utilization function and to optimize the scheduling process for a numerical solver. A program for the experimental schedule is implemented solving a job-shop problem that proved the effectiveness of the proposed algorithm.