Calculation of two-time correlation function and spectrum in quantumoptics.jl

If I have some time-dependent Hamiltonian whose frequency is time dependent not amplitude as described in the example of the documentation in quantumoptics.jl. Then How can I construct two-time correlation function for such time-dependent evolution. Also How to get the spectrum from there. Clearly
such cases do not directly pose any steady-state if evolved with a master equation.