Kaon Decays beyond the Standard Model

We review a new method in order to determine the parameter $\bar{\eta }$ of the Cabibbo-Kobayashi-Maskawa matrix from $K\to {\mu }^{+}{\mu }^{-}$ decays, using interference effects in the time-dependent decay rate. Furthermore, we discuss a new precision relation for the phase-shift of the time-dependent oscillation. The new methodology enables the discovery potential of future time-dependent measurements of $K\to {\mu }^{+}{\mu }^{-}$ decays for physics beyond the Standard Model.