Rtc

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First, the i2c-bcm2708 i2c driver needs to be loaded. As far as I know, in the first few months of the raspberry pi being available, the module was blacklisted and didn't load automatically, but nowadays the module is loaded by default. If it isn't on your system:

sudo modprobe i2c-bcm2708

Load the I2C and RTC drivers as root:

sudo -s
modprobe i2c-dev
modprobe i2c:mcp7941x
echo mcp7941x 0x6f > /sys/class/i2c-dev/i2c-0/device/new_device  # For rev1 RPi
echo mcp7941x 0x6f > /sys/class/i2c-dev/i2c-1/device/new_device  # For rev2 RPi

To write the system time to the RTC (you might need to run this command twice, when you use the RTC for the first time):

hwclock -w

Read out the RTC, and print the date and time to your console:

hwclock

Read out the RTC, and adjust system time:

hwclock -s

To automatically do this on startup, add the following lines to /etc/rc.d/rc.local

modprobe i2c-dev
modprobe i2c:mcp7941x
echo mcp7941x 0x6f > /sys/class/i2c-dev/i2c-0/device/new_device  # For rev1 RPi
echo mcp7941x 0x6f > /sys/class/i2c-dev/i2c-1/device/new_device  # For rev2 RPi
hwclock -s

Source: http://www.element14.com/community/message/63885