  Derivation of two-time second-order correlation function by following
approaches such as stochastic differential equation, coherent-state propagator,
and quasi-statistical distribution function is presented. In the process, the
time dependence of the operators is transferred to the density operator by
making use of trace operation in which the coherent state propagator and
$Q$-function that represent the quantum system under consideration are
expressed in terms of different time parameters. Even though the number of
resulting integrations are found to be large, the accompanying implementation
turns out to be straightforward in view that the associated $c$-number
functions are Gaussian by nature. In relation to the asserted possibility of
rewriting the result of one of the approaches in terms of the other, the
presented derivation is expected to lay a strong foundation for viable
technique of calculating correlations of various moments at different times
that can be deployed in revealing quantum correlations.
