The overall economic efficiency of a quantity-based approach to greenhouse gas mitigation depends strongly on the extent to which such a program provides opportunities for compliance flexibility, particularly with regard to the timing of emissions abatement. Here I consider a program in which annual targets are determined by choosing the optimal time path of reductions consistent with an exogenously prescribed cumulative reduction target and fixed technology set. I then show that if the availability of low-carbon technology is initially more constrained than anticipated, the optimal reduction path shifts abatement toward later compliance periods. For this reason, a rigid policy in which fixed annual targets are strictly enforced in every year yields a cumulative environmental outcome identical to the optimal policy but an economic outcome worse than the optimal policy. On the other hand, a policy that aligns actual prices (or equivalently, costs) with expected prices by simply imposing an explicit price ceiling (often referred to as a "safety valve") yields the opposite result. Comparison among these multiple scenarios implies that there are significant gains to realizing the optimal path but that further refinement of the actual regulatory instrument will be necessary to achieve that goal in a real cap-and-trade system.