DS3231 Ultra-precise Real Time Clock

Features:

• DS3231 I2C real time clock chip (RTC)

• Accuracy: ± 3.5ppm at -40C to +85C (~1 minute per year)
• Power Supply: 2.3 to 5.5 V DC
• Temperature compensated
• Should last at least 8 years on a single CR1632 battery! ( using I2C with 5V power available )
• Built-in 32.768Mhz crystal
• No external crystal or tuning capacitors are required
• Outputs precise year, month, day, hour, minute, second, and morning / afternoon data
• Can output a second output signal of 1Hz
• Optional pull-up resistors R1 and R2 solder points in case your controlling device does not already have them for the I2C bus Since many devices that have I2C buses already have the resistors in place, you most likely won't need to use them with normal use. (4.7K or 10K resistors from SDA and SCL to VCC) 
• Pin Spacing: 0.1" ( 2.54mm)
• Spacing between headers: 0.9" ( 22.86mm)
• 24C32 32K I2C EEPROM memory
• Takes 1 CR1632 coin cell battery (not included)
• Compact design: 1.2" (30mm) diameter
• Current: 200uA (active), 840nA (timekeeping)
• Useful for keeping time even when your projects are off
• Can be chained with other I2C devices on the same serial bus.
• Battery Holder on board
• Includes:
  • RTC Module
  • Header Pins

Useful Reference: http://docs.macetech.com/doku.php/chronodot_v2.0