This work develops a generalized variational framework in which physical evolution is governed by bidirectional temporal boundary constraints imposed at both initial and final temporal boundaries. A combined action–entropy functional is introduced incorporating entropy density into the variational principle. The resulting modified Euler–Lagrange equations describe system evolution under simultaneous dynamical and entropic constraints. Within this framework, syntropic behavior emerges naturally as locally entropy-reducing trajectories corresponding to stationary solutions of the constrained functional. The formulation provides a mathematical basis for analyzing entropy gradients, dissipative structures, and emergent order in nonequilibrium systems. This work extends classical variational mechanics by incorporating entropy-driven constraints and provides a formal framework for studying temporal symmetry and entropy-driven structure formation.