PiRTC

This is a great battery-backed real time clock (RTC) that allows your Raspberry Pi project to keep track of time if the power is lost. Perfect for data-logging, clock-building, time-stamping, timers and alarms, etc. Equipped with PCF8523 RTC, it works great with the Raspberry Pi and has native kernel support.

This RTC will keep the time for about 5 years. The PCF8523 is simple and inexpensive but not a high precision device. It may lose or gain a second or two per day.