  Time has been an elusive concept to grasp. Although we do not yet understand
it properly, there has been advances made in regards to how we can explain it.
One such advance is the Page-Wootters mechanism. In this mechanism time is seen
as an inaccessible coordinate and the apparently passage of time arises as a
consequence of correlations between the subsystems of a global state. Here we
propose a measure that captures the relational character of the mechanism,
showing that the internal coherence is the necessary ingredient for the
emergence of time in the Page-Wootters model. Also, we connect it to results in
quantum thermodynamics, showing that it is directly related to the extractable
work from quantum coherence.
