This is a real time clock module based on DS3231, it has small size, low-cost and extremely high accurate I2C bus real-time clock. The RTC provides information including seconds, minutes, hours, day, date, month and year. The date at the end of month is adjusted automatically for months less than 31 days, including the caliration for leap year. This module can operates either in 12-hour with an AM/PM indicator or 24-hour format. 2 programmable time-of-day alarms and 1 programmable square-wave output are available and address/data are transferred through the I2C bus. At the underside of the board is a 12mm coin-cell battery holder for backup battery using when necessary.The DS3231 is a low-cost I2C RTC, with an integrated temperature-compensated crystal oscillator (TCXO) and crystal. Compared to the DS1302 solution, the DS3231 has many advantages, it function with a VCC ranging from 2.3V to 5.5V and has a battery backup. Unlike the DS1307, the DS3231 also has an integrated crystal, temperature sensor, 2 programmable time-of-day alarms, and a 32.768 kHz output pin to ensure higher accuracy. It is also consists of an EEPROM AT24C32 to supply you 32K EEPROM to store your data.