I will discuss how central extensions of charge algebras in gravitational theories with null boundaries arise from anomalous transformations of the boundary term in the gravitational action. This parallels the way in which the holographic Weyl anomaly appears in AdS/CFT, with the ambiguity in the normalization of the null generator acting analogously to the choice of Weyl frame. This entails understanding the covariant phase space of gravitational subregions which are open Hamiltonian systems. I will describe how to obtain unambiguous expressions for gravitational charges within this formalism, motivated both by AdS/CFT and by the variational principle for gravitational subregions. As an application, I will analyze the near-horizon Virasoro symmetries of black holes and give a systematic derivation of the central charges. I will show that the central extension computed by the charge algebra is directly related to the Bekenstein-Hawking entropy via the Cardy formula.