This is the Real Time Clock (RTC) Module, this little breakout that uses the DS1307 to keep track of the current year, month, day as well as the current time.
The module comes fully assembled and includes a small CR1225 Lithium coin cell battery that will run the RTC for a minimum of 9 years (17 years typical) without an external 5V power supply. The DS1307 RTC is accessed via the I2C protocol. Most microcontroller projects those which log or track data information often require current time and date information to be stored along with the accumulated data. A Real Time Clock (RTC) chip such as DS1307 with a suitable back-up battery can be used to supply the required information. The programming process of the RTC chip (in software) is also very simple and is supported in the majority of programming environments.
The DS1307 serial real-time clock (RTC) is a low power, full binary coded decimal (BCD) clock/calendar plus 56 bytes of NV SRAM. Address and data are transferred serially through an I2C, bidirectional bus. The 24-hour/12-hour format clock/calendar provides seconds, minutes, hours, day, date, month, and year information, including corrections for leap year.