  Let $U_F$ be the Floquet operator of a time periodic hamiltonian $H(t)$.
  For each positive and discrete observable $A$ (which we call a {\em probe
energy}), we derive a formula for the Laplace time average of its expectation
value up to time $T$ in terms of its eigenvalues and Green functions at the
circle of radius $e^{1/T}$. Some simple applications are provided which support
its usefulness.
