Time-based metering is an efficient air traffic management alternative to the more common practice of distance-based metering (or "miles-in-trail spacing"). The efficiency benefit is most pronounced where air traffic flows merge, such as in terminal airspace, at enroute choke points, overhead merge points, or where severe-weather avoidance routes converge - the primary bottlenecks in today's system. To date, the practice of time-based metering in the United States has been confined to arrival airspace, and only in less-constrained regions, such as the West and South, in part due to limitations in the national airspace system infrastructure. Thus, time-based metering has not been available to redress the most critical bottlenecks in the national airspace system and, coincidently, those where time-based metering would be most advantageous. This paper discusses a prototype time-based metering system designed to overcome limitations of the national airspace system to produce a more versatile and scalable time-based metering capability. Results of a live, operational field test are presented which validate that the prototype - the Multi-center Traffic Management Advisor - can extend time-based metering operations beyond the terminal area to improve traffic flow at critical bottlenecks en route and on departure. In the field test, which focused specifically on Philadelphia-bound traffic in four Air Route Traffic Control Centers, airborne delay and airborne holding were significantly less when Multi-center Traffic Management Advisor was in use relative to control periods. The results demonstrate that the Multi-center Traffic Management Advisor is effective in coordinating time-based metering programs among adjacent air traffic control facilities, even in the complex Northeast corridor of the United States. This is a necessary step toward addressing the most critical air traffic bottlenecks in the national airspace system. Potential for nationwide time-based metering and its implications for the next-generation air transportation system also are discussed.