The DS1307 is cheapest and most commonly used 2 wire serial (I2C) Real Time Clock (RTC) chip. This 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 clock/calendar provides seconds, minutes, hours, day, date, month, and year information. The end of the month date is automatically adjusted for months with fewer than 31 days, including corrections for leap year. The clock operates in either the 24-hour or 12-hour format with AM/PM indicator. The DS1307 has a built-in power-sense circuit that detects power failures and automatically switches to the backup supply. Timekeeping operation continues while the part operates from the backup supply. Button cell or commonly known as coin cell is require and must for backup supply, other wise it will not work. If you want to run this chip without backup cell, battery pin should be pulled to ground.
Check this RTC Module