Blue3D(tm) DS3231 AT24C32 I2C Precision Real Time Clock Memory Module Arduino Precise Time Clock with a Temperature-compensated Crystal Oscillator An integrated real time clock module featuring the DS3231 Also has an onboard AT24C32 with 32kb of flash memory Temperature modulated for exceptional time accuracy, less than one minute of error per year Can be used with a rechargeable LIR2032 for battery backup or can be used with a regular CR2032 with diminished protection Easy to connect, just 2 wires for I2C and 2 for power. Use readily available Arduino libraries to get up and running in minutes.