Abstract We propose a mathematical study of Laslett's law, classically used in rock thermochronology. Laslett's law is considered here for time variable thermal histories. This study is based on the numerical analysis of the time equivalent method. It is proved to be an exact method for piecewise constant functions that represent the temperature histories. Error bounds are then obtained for more general temperature histories. Some numerical simulations and comparisons with other classical methods prove the time equivalent method to be well fitted to the large time interval problems in geochronology.