  We show that Berezinskii's classification of the symmetries of Cooper pair
amplitudes holds for driven systems even in the absence of translation
invariance. We then consider a model Hamiltonian for a superconductor coupled
to an external driving potential and, treating the driving potential as a
perturbation, we investigate the corrections to the anomalous Green's function,
density of states, and spectral function. We find that in the presence of an
external drive the anomalous Green's function develops terms that are odd in
frequency and that the same mechanism responsible for these odd-frequency terms
generates additional features in the density of states and spectral function.
