Along with the DS1307 real time clock, the module also has an Atmel 24C32 EEPROM chip which is handy for storing data without worrying about power loss.
DS1307 I2C real time clock chip (RTC)
24C32 32K I2C EEPROM memory
Using CR2032 lithium battery
Other I2C devices can be cascaded
Leads to the DS1307 clock pin provides a clock signal for the microcontroller
Supports LIR2032 rechargeable lithium battery, and with a charging circuit to charge the battery (not included)