  The period 2035-50 considered in the ESA Voyage long-term plan will coincide
with a series of foreseeable advances in the characterization of the stellar
content of the Milky Way. The Gaia mission, combined with large-scale
spectroscopic surveys, is helping to build an unprecedented census in terms of
the astrometric, kinematic and chemical properties of Galactic stellar
populations. Within a decade, precise measurements of such properties will be
available for hundreds of millions of stars. Meanwhile, time-domain surveys
initiated with CoRoT and Kepler/K2 and carried on by space missions such as
TESS and PLATO or ground-based projects like the LSST, will have brought
asteroseismology to a high level of maturity. The combination of precise ages
from asteroseismology with astrometric and spectroscopic data, on large stellar
samples, is allowing Galactic archaeologists to gain new insight into the
assembly history of the Milky Way. Recent breakthroughs --- based on the
detection of solar-like oscillations in tens of thousands of red-giant stars
--- demonstrate the potential of such approach. Therefore, we are convinced
that an all-sky, high-cadence, long-duration stellar variability survey will
become a scientific priority in the 2035-50 period. The Chronos concept
presented here consists in a time-domain extension to Gaia. It will allow for
mass and age estimates for half a million red giants within 1.7 kpc from the
Sun and hence shed a new light on our understanding of the Galactic dynamics
and archaeology. In terms of the targeted pulsators, Chronos will bridge the
gap between PLATO and the LSST by surveying stars all the way from the subgiant
branch to the early AGB. Finally, it will surpass all previous surveys capable
of conducting asteroseismology in terms of the combined sky coverage and
duration of the observations (2 x 3.75 months over the whole sky and >5 years
in the CVZ).
