  The brightness of Neptune is often found to be in accordance with an
adiabatic interior, while the low luminosity of Uranus challenges this
assumption. Here we apply revised equation of state data of hydrogen, helium,
and water and compute the thermal evolution of Uranus and Neptune assuming an
adiabatic interior. For this purpose, we have developed a new planetary model
and evolution code. We investigate the influence of albedo, solar energy
influx, and equations of state of H and He, and water on the cooling time. Our
cooling times of about $\tau_\text{U}=5.1\times 10^9\text{ years}$ for Uranus
and $\tau_\text{N}=3.7\times 10^9\text{ years}$ for Neptune bracket the known
age of the planets of $4.56\times 10^9\text{ years}$ implying that neither
planet's present-day luminosity can be explained by adiabatic cooling. We also
find that uncertainties on input parameters such as the level of irradiation
matter generally more for Uranus than for Neptune. Our results suggest that in
contrast to common assumptions, neither planet is fully adiabatic in the deeper
interior.
