This is a great battery-backed real time clock (RTC) that allows your microcontroller project to keep track of time even if it is reprogrammed, or if the power is lost. Perfect for datalogging, clock-building, time stamping, timers and alarms, etc. The DS1307 is the most popular RTC - but it requires 5V power to work (although we've used it with 5V power and 3.3V logic successfully)

• PCB & header are included
• Plugs into any breadboard, or you can use wires
• Two mounting holes
• Will keep time for 5 years or more

Note: This product does not come with a CR1220 coin cell battery. 

The DS1307 is simple and inexpensive but not a high precision device. It may lose or gain up to 2 seconds a day. 

Adafruit part: 3296