  A method for the precise measurement of drift times for the high-precision
drift chambers used in the TWIST detector is described. It is based on the
iterative correction of the space-time relationships by the time residuals of
the track fit, resulting in a measurement of the effective drift times. The
corrected drift time maps are parametrised individually for each chamber using
spline functions. Biases introduced by the reconstruction itself are taken into
account as well, making it necessary to apply the procedure to both data and
simulation. The described calibration is shown to improve the reconstruction
performance and to extend significantly the physics reach of the experiment.
