  Observing how long a dynamical system takes to return to some state is one of
the most simple ways to model and quantify its dynamics from data series. This
work proposes two formulas to estimate the KS entropy and a lower bound of it,
a sort of Shannon's entropy per unit of time, from the recurrence times of
chaotic systems. One formula provides the KS entropy and is more theoretically
oriented since one has to measure also the low probable very long returns. The
other provides a lower bound for the KS entropy and is more experimentally
oriented since one has to measure only the high probable short returns. These
formulas are a consequence of the fact that the series of returns do contain
the same information of the trajectory that generated it. That suggests that
recurrence times might be valuable when making models of complex systems.
