here i am sending you some of the links it might help you out
what is RTC
http://www.engineersgarage.com/electronic-components/rtc-ds12c887
hw to interface RTC with 8051
http://www.engineersgarage.com/microcontroller/8051projects/interface-rtc-12c887-AT89C51-circuit
project using RTC
http://www.engineersgarage.com/microcontroller/8051projects/alarm-clock-rtc-12c887-AT89C51-circuit