In this talk I will present some results on stricly hyperbolic operators which coefficients are not Lipschitz-continuous with respect to time. In particular if the coefficients are only Zygmund-continuous with respect to time and Lipschitz-continuous with respect to space, then an energy estimate holds, with no loss of derivatives. The content of the talk is part of a joint work with Ferruccio Colombini (Pisa) and Francesco Fanelli (Lyon).