The PCF8523 RTC module from Adafruit is Real-Time Clock (RTC) and calendar optimized for low power consumption. Data is transferred serially via the I2C-bus. Alarm and timer functions are available with the possibility to generate a wake-up signal on an interrupt pin.  

The PCF8523 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

• Provides year, month, day, weekday, hours, minutes, and seconds based on a 32.768 kHz quartz crystal
• 3.3 or 5 V power supply voltage
• Low power consumption
• I2C Serial Interface

PCF8532  Datasheet.

Detailed tutorial from Adafruit including arduino connection and code 

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