We propose to derive the analytical expression of the Hybrid Cramer-Rao Bound HCRB for the joint time delay and channel fading estimation. The time delay is considered as a deterministic parameter and the channel is considered as a complex gaussian random variable. The HCRB expression is computed for both the data-aided (DA) and the non data-aided (NDA) modes. The obtained result allows us to assess the performance of on-line and off-line dynamical Rayleigh channel gains estimation. It also evaluates timing recovery techniques under a time varying fading channel.