Year 2000 - compatible or compliant

It is possible to describe the way a PC reacts to the year 2000 in three ways :-

Compliant - the PC will automatically change the dates held internally to the correct date after 1999-12-31, whether it is powered up through midnight, or is switched off.
The first and all subsequent boot ups after that will also hold the correct date.
Compatible - a PC can be described as compatible with year 2000 if it will produce the correct dates after rebooting, provided the century change is entered manually once only.
A PC that cannot remember the century change even though it has been manually entered, is neither compliant nor compatible.

The following pages describe the reasons behind these different classifications, and look at possible methods of achieving full compliance.