RasClock is a highly accurate real-time clock (RTC) that fits on a Raspberry Pi and includes a battery backup. Battery is not included.
The module is installed directly into the GPIO header at the corner of the Raspberry Pi. It uses the 3.3V, GND SDA and SCL pins. The module stays within the outline of the Raspberry Pi and sits lower than video output so it should fit within most cases.
A 36mA 3V CR1220 battery will keep the time for around two years if the device isn't used and considerably longer if it is.
For setup and detailed instructions, visit the RasClock website.