Disclosed is an operating method of a memory device communicating with a memory controller, which includes receiving a first command from the memory controller, the first command indicating initiation of synchronization of a data clock signal and defining a clock section corresponding to the synchronization, preparing a toggling of the data clock signal during a preparation time period, processing a first data stream based on the data clock signal toggling at a reference frequency, and processing a second data stream based on the data clock toggling at the reference frequency and extended for a period of the defined first clock section.