CONFIG_RTC not only controls the existence of the character special device
/dev/rtc, as its menuconfig location of Device Drivers-->Character
devices-->Enhanced Real Time Clock Support would indicate, it also controls
whether you get the kernel functionality required to perform a proper shutdown
on some systems.  This is highly non-obvious.

If you do not enable this option, it can sabotage the shutdown process in such a
way that your computer will turn itself back on at midnight, unbidden, for no
apparent reason.  At a minimum, this is enough to give someone the screaming
heebie jeebies.  At worst, an unplanned power-up of a critical system could have
dire consequences.

Probably this wants to be two options, one of which gets filed under ACPI with
help text reading "Warning: Disabling this option may result in sleeplessness."

See Bug 4835 and Bug 4896 for details.