  We propose an alternative derivation for the dynamic admittance of a gated
quantum dot connected by a single-channel lead to an electron reservoir. Our
derivation, which reproduces the result of Pr\^{e}tre, Thomas, and B\"{u}ttiker
for the universal charge-relaxation resistance, shows that at low frequencies,
the current leaving the dot lags after the entering one by the Wigner-Smith
delay time. We compute the capacitance when interactions are taken into account
only on the dot within the Hartree-Fock approximation and study the
Coulomb-blockade oscillations as a function of the Fermi energy in the
reservoir. In particular we find that those oscillations disappear when the dot
is fully `open', thus we reconcile apparently conflicting previous results.
