In this paper, we formulate an attack on the relative data alignment scheme used by Phasor Data Concentrators (PDCs) to aggregate and stream phasor measurements. This attack leverages the specification governing the PDC functionality in a Wide Area Measurement System (WAMS). Using a Phasor Measurement Unit (PMU) as an attack surface, an adversary injects an attack vector in the PMU timing to manipulate the PDC functionality and force it to drop phasors received from benign PMUs. We expose the PDC relative data alignment scheme to this attack, formulate the attack surface and vector selection as a Linear Program (LP), and demonstrate the impact of this attack on system observability. Our experimental results on standard IEEE test systems demonstrate the attack semantics and impact. In addition, we present the attack in a case study using hardware-in-the-loop co-simulation environment. The outcome of these tests validate the vulnerability of the PDC aggregation scheme to this attack and its significance in targeting system observability.