  We present a gauge--theoretical derivation of the notion of time, suitable to
describe the Hamiltonian time evolution of gravitational systems. It is based
on a nonlinear coset realization of the Poincar\'e group, implying the time
component of the coframe to be invariant, and thus to represent a metric time.
The unitary gauge fixing of the boosts gives rise to the foliation of spacetime
along the time direction. The three supressed degrees of freedom correspond to
Goldstone--like fields, whereas the remaining time component is a Higgs--like
boson.
