  A general method is introduced for verifying multitime quantum correlations
through the characteristic function of the time-dependent P functional that
generalizes the Glauber-Sudarshan P function. Quantum correlation criteria are
derived which identify quantum effects for an arbitrary number of points in
time. The Magnus expansion is used to visualize the impact of the required time
ordering, which becomes crucial in situations when the interaction problem is
explicitly time dependent. We show that the latter affects the
multi-time-characteristic function and, therefore, the temporal evolution of
the nonclassicality. As an example, we apply our technique to an optical
parametric process with a frequency mismatch. The resulting
two-time-characteristic function yields full insight into the two-time quantum
correlation properties of such a system.
