  In quantum theory it is possible to explain time, and dynamics, in terms of
entanglement. This is the timeless approach to time, which assumes that the
universe is in a stationary state, where two non-interacting subsystems, the
clock and the rest, are entangled. As a consequence, by choosing a suitable
observable of the clock, the relative state of the rest of the universe evolves
unitarily with respect to the variable labelling the clock observable's
eigenstates, which is then interpreted as time. This model for an evolution
without evolution (Page and Wootters, 1983), albeit elegant, has never been
developed further, because it was criticised for generating severe ambiguities
in the dynamics of the rest of the universe. In this paper we show that there
are no such ambiguities, we also update the model, making it amenable to
possible new applications.
