We analyze two-time correlators as the most natural characteristic of a
propagating quadrature-squeezed field in the transient regime. The considered
system is a parametrically driven resonator with a time-dependent drive. Using
a semiclassical approach derived from the input-output theory, we develop a
technique for calculation of the two-time correlators, which are directly
related to fluctuations of the measured integrated signal. While in the steady
state the correlators are determined by three parameters (as for the
phase-space ellipse describing a squeezed state), four parameters are necessary
in the transient regime. The formalism can be generalized to weakly nonlinear
resonators with additional coherent drive. We focus on squeezed microwave
fields relevant to the measurement of superconducting qubits; however, our
formalism is also applicable to optical systems. The results can be readily
verified experimentally.