  This paper concerns the instantaneous frequency (IF) of continuous-time,
zero-mean, complex-valued, proper, mean-square differentiable nonstationary
Gaussian stochastic processes. We compute the probability density function for
the IF for fixed time, which extends a result known for wide-sense stationary
processes to nonstationary processes. For a fixed time the IF has either zero
or infinite variance. For harmonizable processes we obtain as a byproduct that
the mean of the IF, for fixed time, is the normalized first order frequency
moment of the Wigner spectrum.
