An electronic musical apparatus is of a data processing type which boots up when a power is supplied and shuts down when the power is removed. The apparatus comprises an internal clock which is not backed up by an uninterruptible power supply and stops counting time when the power supply to the apparatus is shut down. Upon boot of the apparatus or as preparation for execution of processing which needs a reference time-of-day clock counting, the apparatus acquires time-of-day clock counting information, and corrects the internal clock to run with the time-of-day counting based on the time-of-day information acquired externally, so that the apparatus can execute processing which needs a reference time-of-day clock counting, even though it is not equipped with a backup power supply for the internal clock. In case the acquisition of time-of-day information has not been successful, the execution of such processing will be prohibited, or the internal clock may be corrected based on a default time-of-day or on a user-set time-of-day.