  We present the first study of robustness of systems that are both timed as
well as reactive (I/O). We study the behavior of such timed I/O systems in the
presence of "uncertain inputs" and formalize their robustness using the
analytic notion of Lipschitz continuity. Thus, a timed I/O system is
K-(Lipschitz) robust if the perturbation in its output is at most K times the
perturbation in its input. We quantify input and output perturbation using
"similarity functions" over timed words such as the timed version of the
Manhattan distance and the Skorokhod distance. We consider two models of timed
I/O systems --- timed transducers and asynchronous sequential circuits. While
K-robustness is undecidable even for discrete transducers, we identify a class
of timed transducers which admits a polynomial space decision procedure for
K-robustness. For asynchronous sequential circuits, we reduce K-robustness
w.r.t. timed Manhattan distances to K-robusness of discrete letter-to-letter
transducers and show PSPACE-compeleteness of the problem.
