  The fractional Fokker-Planck equation for subdiffusion in time-dependent
force fields is derived from the underlying continuous time random walk. Its
limitations are discussed and it is then applied to the study of subdiffusion
under the influence of a time-periodic rectangular force. As a main result, we
show that such a force does not affect the universal scaling relation between
the anomalous current and diffusion when applied to the biased dynamics: in the
long time limit subdiffusion current and anomalous diffusion are immune to the
driving. This is in sharp contrast with the unbiased case when the subdiffusion
coefficient can be strongly enhanced, i.e. a zero-frequency response to a
periodic driving is present.
