  Nuclear chronometer, which predicts the ages of the oldest stars by comparing
the present and initial abundances of long-lived radioactive nuclides, provides
an independent dating technique for the cosmos. A new nuclear chronometer
called Th-U-X chronometer is proposed, which imposes stringent constraints on
the astrophysical conditions in the $r$-process simulation by synchronizing the
previous Th/X, U/X and Th/U chronometers. The astrophysical uncertainties of
nuclear chronometer are significantly reduced from more than $\pm2$ billion
years to within 0:3 billion years by the Th-U-X chronometer. The proposed
chronometer is then applied to estimate the ages of the six metal-poor stars
with observed uranium abundances, and the predicted ages are compatible with
the cosmic age 13.8 billion years predicted from the cosmic microwave
background radiation, but in contradictory with the new cosmic age 11.4 billion
years from the gravitational lenses measurement.
