Quantum probes of gravity are commonly described in terms of effective interactions between systems or their coupling to a weak gravitational field. However, the interpretation of these descriptions requires an account of the physical degrees of freedom and observables involved. In this talk, I will focus on the challenge of describing clocks and time measurements consistently with the symmetries of gravity. I will discuss how this challenge arises in weak-field gravity, the role of constraints and gauge fixing, and the implications for the interpretation of quantum experiments. In particular, I will ask to what extent such experiments can distinguish between different descriptions of gravity, including perturbative and non-perturbative approaches.