A clock device is held by a moving body, and receives a modulated wave from two electromagnetic fields generated by an antenna formed in a predetermined shape and disposed on a path. The clock device demodulates the modulated wave and acquires unique identification information. The clock device determines a non-detection period after the identification information is acquired. The clock device acquires a reference time corresponding to a middle of the non-detection period, and specifies the acquired reference time as a pass time when the moving body has passed through a point where the antenna is disposed.