It has been demonstrated that delay fault coverage loss could be significant if improper propagation paths are used. This occurs when the delay test pair of a target propagation path cannot be effectively generated by an ATPG tool, or when stuck-at test patterns are used as transition (or gate) delay test patterns. In this work, an efficient method is proposed to reduce the amount of fault coverage loss by using multiple observation times. The basic idea is to offset the shorter propagation paths (really used) by tightening the observation times. Given a probability distribution of defect sizes and a set of slack differences, this method is able to locate several observation times that result in small fault coverage loss.