The DS1307 serial real-time clock (RTC) from Adafruit is a low power clock/calendar. The clock/calendar provides seconds, minutes, hours, day, date, month, and year information. The end of the month date is automatically adjusted for each month (31 days or less) 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. It can be used in many applications such as data logging, clock-building, time stamping, timers, calendars and alarms. 

Features

• Real-Time Clock Counts Seconds, Minutes, Hours, Date of the Month, Month, Day of the Week, and Year with Leap-Year Compensation Valid Up to 2100
• 5 V power supply voltage
• - 40 ℃ to + 85℃ temperature range
• Low power consumption
• I2C Serial Interface

 DS1307  Datasheet.

Detailed tutorial from Adafruit including arduino connection and code 

Detailed tutorial from Adafruit including arduino connection and code in pdf format