The DS1307 Real Time Clock developed by one of our designers: Waiman. The module comes fully assembled and pre-programmed with the current time (ok, so it's our current time - MST). The included Lithium coin cell battery (CR1225 41mAh) will run the module for a minimum of 9 years (17 years typical) without external 5V power. The DS1307 is accessed via the I2C protocol.
Except the DS1307 real time clock, the module also integrate the I2C EEPROM chip(24C32) and the DS18B20 sensor interface. The I2C EEPROM makes it possible to save the sensor data easily without spending too much microcontroller source. And the DS18B20 interface with the pull-up resistor embeded will be helpful to extend temperature monitoring feature for your project.