We present a unified geometric framework for describingboth the Lagrangian and Hamiltonian formalisms of regularand non-regular time-dependent mechanical systems, which isbased on the approach of Skinner and Rusk (1983). Thedynamical equations of motion and their compatibility andconsistency are carefully studied, making clear that allthe characteristics of the Lagrangian and the Hamiltonianformalisms are recovered in this formulation. As anexample, it is studied a semidiscretization of thenonlinear wave equation proving the applicability of theproposed formalism.