In the Large High Altitude Air Shower Observatory (LHAASO), one square kilometer array (KM2A), with 5242 electromagnetic particle detectors (EDs) and 1171 muon detectors (MDs), is designed to study ultra-high energy gamma-ray astronomy and cosmic ray physics. The first installed 33 EDs array has been running steadily since February 2018. A set of offline detector self-calibration and data quality monitoring, which relies on the measurement of charged particles within the extensive air showers, has been applied in the data analysis. In this work, the experimental setup and the offline time calibration are described. Finally, observations of the expected moon and sun’s shadows of cosmic rays verify the time calibration method.