Aus der Doku zu make (menu)config:

 If you enable this option and create a character special file
x
  x /dev/rtc with major number 10 and minor number 135 using mknod ("man
x
  x mknod"), you will get access to the real time clock built into your
x
  x computer. Every PC has such a clock built in. It can be used to
x
  x generate signals from as low as 1Hz up to 8192Hz, and can also be
x
  x used as a 24 hour alarm.  It reports status information via the file
x
  x /proc/rtc and its behaviour is set by various ioctls on
x
  x /dev/rtc. People running SMP (= multiprocessor) versions of Linux
x
  x should enable this option to read and set the RTC clock in a SMP
x
  x compatible fashion. If you think you have a use for such a device
x
  x (such as periodic data sampling), then say Y here, and go read the
x
  x file Documentation/rtc.txt for details. 
x
  x