  In ground-based extensive air shower experiments, the direction and energy
are reconstructed by measuring the relative arrival time of secondary
particles, and the energy they deposit. The measurement precision of the
arrival time is crucial for determination of the angular resolution. For this
purpose, we need to obtain a precise relative time offset for each detector,
and to apply the calibration process. The time offset is associated with the
photomultiplier tube, cable, relevant electronic circuits, etc. In view of the
transit time through long cables being heavily dependent on the ambient
temperature, a real-time calibration method for the cable transit time is
investigated in this paper. Even with a poor-resolution time-to-digital
converter, this method can achieve high precision. This has been successfully
demonstrated with the Front-End-Electronic board used in the Daya Bay neutrino
experiment.
