  The description of an observer's measurement in general relativity and the
standard model of particle physics is closely related to the spacetime metric.
In order to understand and interpret measurements, which test the metric
structure of the spacetime, like the classical Michelson-Morley, Ives-Stilwell,
Kennedy-Thorndike experiments or frequency comparison experiments in general,
it is necessary to describe them in theories, which go beyond the Lorentzian
metric structure. However, this requires a description of an observer's
measurement without relying on a metric. We provide such a description of an
observer's measurement of the fundamental quantities time and length derived
from a premetric perturbation of Maxwell's electrodynamics and a discussion on
how these measurements influence classical relativistic observables like time
dilation and length contraction. Most importantly, we find that the
modification of electrodynamics influences the measurements at two instances:
the propagation of light is altered as well as the observer's proper time
normalization. When interpreting the results of a specific experiment, both
effects cannot be disentangled, in general, and have to be taken into account.
