PURPOSE: To make an external standard clock of a computer unnecessary, by constituting so that a service interruption time is counted at the time of service interruption of a computer, adding a count value and a service interruption generation time which has been saved in a memory device, after an electric power supply has been returned, and calculating a present time.

CONSTITUTION: When an operating electric power supply VS1 from the outside is disconnected due to service interruption, an operating electric power supply switching circuit 210 changes over an operating electric power supply to a battery 208, and an electric power supply VS2 is supplied to each circuit in an interval timer module 23. A signal of rise-up in case when the electric power supply is changed over is delayed for a constant time, and after that, is waveform-shaped by a one shot circuit 212, and is inputted to an AND gate 214. In the gate 214, the fate is opened when the electric power supply VS1 is disconnected, a clear signal is sent to a time counting counter 203, makes contents of the counter 203 "0", and after that, counts a clock of a clock generator 202 over again. When the electric power supply is released, a count value of the counter 203 is stored in a read register 204 and is sent to an operation control part 21 by a signal from the control part 21, and is added to a service interruption generation time which has been saved in a memory device 22.