Visible light communications (VLC) in general and resource allocation for VLC networks particularly have gained lots of attention recently. In this work, we consider the resource allocation problem of a VLC downlink transmission system employing dynamic time division multiple access where time and power variables are tuned to maximize spectral efficiency (SE). As for the operational conditions, we impose constraints on the average optical intensity, the energy budget and the quality-of-service. To solve the non-convex problem, we transform the objective function into a difference of concave functions by solving a second order differential inequality. Then, we propose a low-complexity algorithm to solve the resource allocation problem. Finally, we show by simulations the SE performance gains achieved by optimizing time and power allocation over the initial total power minimization solution for the considered system.