For our RTC to work, we will have to patch the kernel sources to add support for PCF2127A and compile with support for I2C. I recommend following the steps explained in this previous article where the kernel was compiled on an x86-based PC since it will be much faster than compiling the kernel on the RaspberryPi itself.

We proceed with the patching of the sources:

We compile the kernel:

Device Drivers -> I2C support -> I2C Hardware Bus support -> BCM2708 BSC
Device Drivers -> Real Time Clock -> Philips PCF2127A (only I2C)

We must indicate which address within the I2C bus it has in RTC(0x51) and which module should manage it:

We assign the date manually and transfer it to the RTC:

If we want to read the date saved in the RTC, just:

If we want to read the date from the RTC and configure the OS:

For everything to work correctly, the timezone must be configured correctly:

• Gentoo:
  

  cp /usr/share/zoneinfo/Europe/Madrid /etc/localtime
  echo “Europe/Madrid” > /etc/timezone

• Debian:
  

  dpkg-reconfigure tzdata

At each boot, we will read the date from the RTC and configure it in the OS:

• Gentoo:
  

  vi /etc/local.d/RTC.start

  echo pcf2127a 0x51 > /sys/class/i2c-adapter/i2c-1/new_device
  sleep 2
  hwclock -s &
  

  chmod 700 /etc/local.d/RTC.start

• Debian:
  

  vi /etc/rc.local

  echo pcf2127a 0x51 > /sys/class/i2c-adapter/i2c-1/new_device
  sleep 2
  hwclock -s &
  

Now, even if we turn off the rasp and remove the power, the date will not be lost unless the RTC button battery runs out.