The period variations of rotating, pulsating and eclipsing variable stars bear valuable astrophysical information about the presence of companions, evolutionary effects, and the inner structure of the stars. This talk described a universal method for de-trending and re-scaling precise photometric data (Kepler, MOST, CoRoT, OGLE, …) appropriate for period-change diagnostics of periodic variables. We demonstrated the potential of the method by analysing the period variability of one of the newly-identified Kepler magnetic chemically-peculiar (mCP) stars. We showed that, surprisingly, our target star displays near-sinusoidal changes in its observed light-variations, with a period of 2.85(6) years, which is apparently the result of the presence of a nearby stellar companion. The expected long-term changes of the rotational period, as have been observed in several mCP stars, have not been identified among the sample of Kepler mCP stars.