  Uncertainty relations play a crucial role in quantum mechanics. Well-defined
methods exist for the derivation of such uncertainties for pairs of
observables. Other approaches also allow the formulation of time-energy
uncertainty relations, even though time is not an operator in standard quantum
mechanics. However, in these cases, different approaches are associated with
different meanings and interpretations for these relations. The one of interest
here revolves around the idea of whether quantum mechanics inherently imposes a
fundamental minimum duration for energy measurements with a certain precision.
In our study, we investigate within the Page and Wootters timeless framework
how energy measurements modify the relative "flow of time" between internal and
external clocks. This provides a unified framework for discussing the subject,
allowing us to recover previous results and derive new ones. In particular, we
show that the duration of an energy measurement carried out by an external
system cannot be performed arbitrarily fast from the perspective of the
internal clock. Moreover, we show that during any energy measurement the
evolution given by the internal clock is non-unitary.
