Floquet formulation of time-dependent density-functional theory is revisited in light of its recent criticism [N.T. Maitra, K. Burke, Chem. Phys. Lett. 359 (2002) 237]. It is shown that Floquet theory is well founded and its criticism has overlooked important points of both the Runge-Gross formalism and Floquet formulation itself. We substantiate our analysis by examples similar to that considered by Maitra and Burke. (c) 2006 Elsevier B.V. All rights reserved.