  Thanks to space-borne experiments of cosmic-ray (CR) detection, such as the
AMS and PAMELA missions in low-Earth orbit, or the Voyager-1 spacecraft in the
interstellar space, a large collection of multi-channel and time-resolved CR
data has become available. Recently, the AMS experiment has released new
precision data, on the proton and helium fluxes in CRs, measured on monthly
basis during its first six years of mission. The AMS data reveal a remarkable
long-term behavior in the temporal evolution of the proton-to-helium ratio at
rigidity $R = p/Z <$ 3 GV. As we have argued in a recent work, such a behavior
may reflect the transport properties of low-rigidity CRs in the inteplanetary
space. In particular, it can be caused by mass/charge dependence of the CR
diffusion coefficient. In this paper, we present our developments in the
numerical modeling of CR transport in the Milky Way and in the heliosphere.
Within our model, and with the help of approximated analytical solutions, we
describe in details the relations between the properties of CR diffusion and
the time-dependent evolution of the proton-to-helium ratio.
