The present invention teaches and claims an image forming apparatus using a time measuring apparatus to accurately measure the time in which the image forming apparatus stays in one or more modes. The image forming apparatus comprises a first CPU which outputs a pulse signal to a receiving second CPU. The second CPU calculates, by using the received pulse signal, a timer error of a second clock signal. The time measuring apparatus measures time in one or more modes based on the second clock signal and then corrects the measured time using the calculated timer error.