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Phasor Measurement Units (PMUs) are being deployed on electric power grids to monitor their static and dynamic behavior. The information they gather is important to detect problems on these systems and to prevent blackouts. The PMUs require time synchronization of the data taken across the grids. This is done using Coordinated Universal Time (UTC) usually derived from the Global Positioning System (GPS). This paper describes the basic operating principles of PMUs and their time requirements to estimate absolute phase angles. A NIST PMU calibration system that tests PMU performance necessary to meet the IEEE synchrophasor standard requirements and the traceability of these measurements is also discussed.