  Direct observation of gravitational waves from binary black hole (BBH)
mergers has made it possible to test the laws of black-hole thermodynamics
using real astrophysical sources. These tests rely on accurate and unbiased
parameter estimates from the pre- and post-merger portions of a signal. Due to
numerical complications, previous analyses have fixed the sky location and
coalescence time when independently estimating the parameters of the pre- and
post-merger signal. Here we overcome the numerical complications and present a
novel method of marginalizing over sky location and coalescence time. Doing so,
we find that it is not possible to model only the pre- or post-merger portions
of the signal while marginalizing over timing uncertainty. We surmount this
problem by simultaneously yet independently modelling the pre- and post-merger
signal, with only the sky location and coalescence time being shared between
the models. This allows us to marginalize over all parameters. We use our
method to measure the change in area $\Delta A_{\rm measured} = A_f - A_i$
between the final and initial black holes in the BBH merger GW150914. To
measure the final black hole's area $A_f$ we do an analysis using quasi-normal
modes (QNMs) to model the post-merger signal, and another analysis using the
post-merger portion of an inspiral-merger-ringdown (IMR) template. We find
excellent agreement with expectations from General Relativity. The Hawking area
theorem (which states that $A_f \geq A_i$) is confirmed to $95.4\%$ and
$99.5\%$ confidence using the QNM and IMR post-merger models, respectively.
Both models yield $\Delta A_{\rm measured} / \Delta A_{\rm expected} \sim 1$,
where $\Delta A_{\rm expected}$ is the expected change in area derived from
fits to numerical relativity simulations.
