  The question about the appearance of time in the semiclassical limit of
quantum gravity continues to be discussed in the literature. It is believed
that a temporal Schrodinger equation for matter fields on the background of a
classical gravitational field must be true. To obtain this equation, the Born -
Oppenheimer approximation for gravity is used. However, the origin of time in
this equation is different in works of various authors. For example, in the
papers of Kiefer and his collaborators, time is a parameter along a classical
trajectory of gravitational field; in the works of Montani and his
collaborators the origin of time is introducing the Kuchar - Torre reference
fluid; in the extended phase space approach the origin of time is the
consequence of existing of the observer in a fixed reference frame. We discuss
and compare these approaches. To make the calculations transparent, we
illustrate them with a model of a closed isotropic universe. In each approach,
one obtains some Schrodinger equation for matter fields with quantum
gravitational corrections, but the form of the equation and the corrections
depend on additional assumptions which are rather arbitrary. None of the
approaches can explain how time had appeared in the Early Universe, since it is
supposed that classical gravity and, therefore, classical spacetime had already
come into being.
