This is Real Time Clock module integrated with RTC DS1307 IC and a back up CR2032 battery holder (Battery is not included). And it also comes with additional EEPROM memory IC, AT24C32 plus ready soldering pad to add digital temperature sensor DS18B20 (not supplied on the board) which are connected via I2C bus on a 27mm x 28mm x 8.4mm board.
DS1307 is a low-power real-time clock chip with 56 bytes of non-volatile RAM, full BCD code clock and calendar. The address and data will be communicated via a two-wire bidirectional serial bus (I2C), and the IC offers information such as seconds, minutes, hours, etc., and the number of days in each month can be automatically adjusted. Besides, there is a compensation function for leap year. AM / PM flag is to determine whether the clock operates in 24-hour or 12-hour mode, and there is a built-in power-sense circuit in the chip with powering down detecting and battery switching functions.
AT24Cxx series of EEPROM are produced by Microchip. AT24C32 on this board offers 32K bit (8192 bytes) of CMOS EEPROM memory with I2C bus data transfer protocol, which are electrically erasable, programmable for self-timed writing cycle (including auto erase time of no more than 10ms, typical time of 5ms) . It supports 1.8V to 5V power supply, which can be erased for one million times, and the data can be kept for 100 years. When using a 5V power supply, the I2C clock speed can reach 400 KHz.
The optional temperature sensor, DS18B20 adopts single-bus technology, which can effectively reduce external interference and improve the measurement accuracy. Meanwhile, it can convert the measured temperature directly into a serial digital signal for microcontroller to process.
Features
DS1307 I2C Real-Time Clock IC (RTC)
AT24C32 32KBit (8KByte) I2C EEPROM memory  IC
CR2032 Back Up battery holder (battery not included)
Fresh new CR2032 can provides 1 year of timing for DS1307
Breaks out DS1307 clock pin to offer clock signals for the microcontroller.
Can be cascaded to other I2C devices.
3.3V and 5.0V compatible.
Breakout I2C and Power pins for external microcontroller interface, example Arduino or PIC
Dimension: 27mm * 28mm * 8.4mm