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Increases in air traffic volume and complexity will create substantial challenges for our nation's air traffic management (ATM) system over the coming decades. The next generation air transportation system (NextGen) is a response to those challenges, a quantum leap forward that integrates human operators, automation, and aircraft within a net-centric and collaborative environment based on precise performance-based operations and common information sharing. Performance-based ATM is a mid-term step toward NextGen that has been demonstrated in a simulation environment and shows great promise as a scalable solution that is necessary, desirable, and achievable. Performance-based ATM is a set of capabilities, validation activities, and an operational concept that is firmly aligned with the NextGen concept and represents an affordable and realistic path to its mid-term implementation. Performance-based ATM integrates advanced flight deck communications, navigation, and surveillance (CNS) capabilities with improved ground automation, procedures, and flow strategies to improve predictability and overall ATM system performance. In such an environment, time based metering is an essential element in precisely regulating traffic flows across all domains. To date, the application of time based metering in arrival airspace has been accomplished using the traffic management advisor (TMA). Time based metering is an efficient alternative to the current practice of using miles-in-trail metering and becomes particularly beneficial when strategic resolutions can be applied to problems of contention at constrained resources. The performance-based ATM concept for time based metering in part drives the need for more accurate trajectories to be used for scheduling meter times and for strategic detection of metering problems. This paper presents the time based metering concept and focuses on the en route capabilities is needed to support that concept and identifies significant issues and ar- eas where further research is needed.