  We have developed a new laser-based time calibration system for the MEG II
timing counter dedicated to timing measurement of positrons. The detector
requires precise timing alignment between $\sim\,$500 scintillation counters.
In this study, we present the calibration system which can directly measure the
time offset of each counter relative to the laser-synchronized pulse. We
thoroughly tested all the optical components and the uncertainty of this method
is estimated to be 24 ps. In 2017, we installed the full system into the MEG II
environment and performed a commissioning run. This method shows excellent
stability and consistency with another method. The proposed system provides a
precise timing alignment for SiPM-based timing detectors. It also has potential
in areas such as TOF-PET.
