This project is a Real time clock shield for Arduino based on DS1307 realtime (RTC) integrated circuit. It is a fairly accurate clock that can keep time when not powered if you add a 3V battery. Data is transfered to microcontroller through I²C bidirectional interface. It is connected to Arduino at pins 4 and 5. Check schematic on the link below.
Arduino DS1307 Real Time Clock Shield - [Link]