The extinct p-process nuclide 146Sm serves as an astrophysical and geochemical chronometer through measurements of isotopic anomalies of its alpha -decay daughter 142Nd. Based on analyses of 146Sm/147Sm alpha -activity and atom ratios, we determined the half-life of 146Sm to be 68 plus or minus 7 (1 sigma ) million years, which is shorter than the currently used value of 103 plus or minus 5 million years. This half-life value implies a higher initial 146Sm abundance in the early solar system, (146Sm/144Sm)0 = 0.0094 plus or minus 0.0005 (2 sigma ), than previously estimated. Terrestrial, lunar, and martian planetary silicate mantle differentiation events dated with 146Sm-142Nd converge to a shorter time span and in general to earlier times, due to the combined effect of the new 146Sm half-life and (146Sm/144Sm)0 values.