  A commonly adopted relational account of time evolution in
generally-covariant systems, and more specifically in quantum cosmology, is
argued to be unsatisfactory, insofar as it describes evolution relative to
observed readings of a clock that does not exist as a bona fide observable
object. A modified strategy is proposed, in which evolution relative to the
proper time that elapses along the worldline of a specific observer can be
described through the introduction of a `test clock', regarded as internal to,
and hence unobservable by, that observer. This strategy is worked out in detail
in the case of a homogeneous cosmology, in the context of both a conventional
Schrodinger quantization scheme, and a `polymer' quantization scheme of the
kind inspired by loop quantum gravity. Particular attention is given to
limitations placed on the observability of time evolution by the requirement
that a test clock should contribute only a negligible energy to the Hamiltonian
constraint. It is found that suitable compromises are available, in which the
clock energy is reasonably small, while Dirac observables are reasonably
sharply defined.
