  We consider the time-dependent analysis of rare $B_d$ and $B_s$ decays
mediated by $b\to s \nu \bar \nu$ transitions. The inclusion of time evolution
allows us to construct novel observables with specific sensitivity to CP-odd
phases in these processes. The sensitivity to CP violation of corresponding
time-integrated measurements in presence of flavor-tagging is also explored. We
provide precise predictions for these observables in the SM and explore their
sensitivity to new CP-violating NP contributions at present and planned future
$B$-physics experiments. As such, these observables provide unique probes of CP
violation in $b \to s \nu \bar \nu$ transitions.
