All PCs (even Alpha machines) are equipped with a real time clock. This clock keeps the date when the PC is turned off.
The hardware clocks has its own power supply (battery, capactior etc.) so that it continues to keep track of the time even if the system is powered of or unplugged.
Other names for the hardware clock are: RTC, BIOS clock or CMOS clock. The kernel sometimes names it persistent clock.
Hardware clocks are usually too slow or too fast by a more or less constant amount of time per day. This amount of time is called the systematic drift. hwclock keeps track of the necessary information in the adjtime file (/etc/adjtime)
The hardware clock does not have the concept of time standard (localtime or UTC) nor of daylight saving time (DST), it does not keep track of the time in a timezone.
Generating signals
The clock can be used to generate signals from 2 HZ up to 8192 HZ in increments of powers of two.