If you say Y here and create a character special file /dev/rtc with
major number 10 and minor number 135 using mknod ("man mknod"), you
will get access to the real time clock (or hardware clock) built
into your computer.

Every PC has such a clock built in. It can be used to generate
signals from as low as 1Hz up to 8192Hz, and can also be used
as a 24 hour alarm. It reports status information via the file
/proc/driver/rtc and its behaviour is set by various ioctls on
/dev/rtc.

If you run Linux on a multiprocessor machine and said Y to
"Symmetric Multi Processing" above, you should say Y here to read
and set the RTC in an SMP compatible fashion.

If you think you have a use for such a device (such as periodic data
sampling), then say Y here, and read Documentation/rtc.txt
for details.

To compile this driver as a module, choose M here: the
module will be called rtc.

source code:

drivers/char/rtc.c