A system for real time clock (RTC) calibration includes: a timer counter; a clock generator; and a clock calibration unit coupled between the clock generator and the timer counter. The clock calibration unit receives calibration parameters comprising an average calibration value, a remainder calibration value and a calibration period, counts a plurality of clock cycles generated by the clock generator, calibrates a number of the counted clock cycles according to the average calibration value and remainder calibration value in the calibration period, and increments the timer counter by one second when a predetermined number of clock cycles have been reached.