We study the computational power of Piecewise Constant Derivative (PCD) systems. PCD systems are dynamical systems defined by a piecewise constant differential equation and can be considered as computational machines working on a continuous space with a continuous time. We show that the computation time of these machines can be measured either as a discrete value, called discrete time, or as a continuous value, called continuous time. We relate the two notions of time for general PCD systems. We prove that general PCD systems are equivalent to Turing machines and linear machines in finite discrete time. We prove that the languages recognized by purely rational PCD systems in dimension $d$ in finite continuous time are precisely the languages of the $d-2^{th}$ level of the arithmetical hierarchy. Hence the reachability problem of purely rational PCD systems of dimension $d$ in finite continuous time is $\Sigma_{d-2}$-complete.