  Given the tension between the values of the Hubble parameter $H_0$ inferred
from the cosmic microwave background (CMB) and from supernovae, attention is
turning to time delays of strongly lensed quasars. Current time-delay
measurements indicate a value of $H_0$ closer to that from supernovae, with
errors on the order of a few percent, and future measurements aim to bring the
errors down to the subpercent level. Here we consider the uncertainties in the
mass distribution in the outskirts of the lens. We show that these can lead to
errors in the inferred $H_0$ on the order of a percent and, once accounted for,
would correct $H_0$ upward (thus increasing slightly the tension with the CMB).
Weak gravitational lensing and simulations may help to reduce these
uncertainties.
