A method and a system for validating clock skews during a hierarchical static timing analysis of a chip or multi-chip package. Each pair of clock inputs of a hierarchical module bounds the allowable clock skew, creating new relative constraints on clock input arrival times propagated to those clock inputs. One embodiment is based on asserted arrival times and a maximum of computed slack values at said clock inputs, while a second embodiment is based on asserted arrival times and a minimum of downstream test slack values. The method further converts module clock assertions into a set of relative timing constraints to allow a hierarchical timing sign-off even in circumstances where absolute timing arrivals are not totally known at the time of module analysis.