  The Hamiltonian constraint system is the canonical formulation of a physical
system with a Hamiltonian constrained to vanish. In terms of the canonical
variables, we define what we call reference observable, with respect to which
other observables evolve. We study if it plays the role of time. As simple
examples, we study the theories of non-relativistic and relativistic particles.
We outline an application of the idea to general relativity.
