Abstract: Time-indicating data which is continuously updated by an add-on clock/calendar circuit is conveyed to a general-purpose microcomputer to set the clock mechanism of the computer which otherwise is updated only when the computer is operated. Such time-indicating data is generated independently of operation of the computer and is conveyed to the computer by way of its internal peripheral interface circuit normally dedicated for transferring data from the computer through an output port of the computer. The clock/calendar circuit is connected in parallel with the internal peripheral interface circuit and normally does not interfere with normal transfer of data from the computer to the printer. Consequently, a continuously updated clock/calendar circuit is made available for use by the computer without using an expansion slot.