  Certain dynamical black hole solutions can be mapped to static spacetimes by
conformal metric transformations. This mapping provides a physical link between
the conformal Killing horizon of the dynamical black hole and a Killing horizon
of the static spacetime. We show how this conformal relation can be used to
derive thermodynamic properties for the dynamical black holes. Although these
horizons are defined quasi-locally and can be located by local experiments,
they are distinct from other popular notions of quasi-local horizons such as
apparent horizons. Thus in the dynamical Vaidya spacetime describing constant
accretion of null dust, the conformal Killing horizon, which is null by
construction, is the natural horizon to describe the black hole.
