  The standard formulation of quantum theory relies on a fixed space-time
metric determining the localisation and causal order of events. In general
relativity, the metric is influenced by matter, and is expected to become
indefinite when matter behaves quantum mechanically. Here, we develop a
framework to operationally define events and their localisation with respect to
a quantum clock reference frame, also in the presence of gravitating quantum
systems. We find that, when clocks interact gravitationally, the time
localisability of events becomes relative, depending on the reference frame.
This relativity is a signature of an indefinite metric, where events can occur
in an indefinite causal order. Even if the metric is indefinite, for any event
we can find a reference frame where local quantum operations take their
standard unitary dilation form. This form is preserved when changing clock
reference frames, yielding physics covariant with respect to quantum reference
frame transformations.
