Some people run a PC test by setting the date at Dec. 31, 1999, 11:55 p.m. (or 23:55), and then see if it rolls over. (Don't try this unless you have backed up your computer's drtives. You could lose data if the test fails.) When it doesn't roll over, they correct it on screen. Great. But after you turn it off in 2000 or later, it will reboot to 1980 or 1984. You must add the new date every time, forever. Also, this doesn't solve your software problem. Think: "spreadsheets."
Here is the assessment by the National Institute of Standards and Technology.
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Now, the industry faces the problem of adding those two century digits back into the date field in order to keep software running and producing correct output. The problem, however, is not isolated to software. Hardware will also cause difficulties for system administrators and chief information officers. System clocks on virtually every personal computer will wind up with corrupted dates on January 1, 2000.
In some cases, the date will appear to roll over to the correct date, but when the machine is turned off and then back on for the next session, an odd date will have taken its place. It may appear as January 1, 1980; January 4, 1980; January 1, %000; or some other combination of characters, all of which will produce erroneous results. The dilemma is not limited to personal computers. Some workstations, minicomputers, mainframes, elevators, and automobile central computers will fall victim to the insidious problem. In most cases, software patches can alleviate the problem to a more-or-less livable extent, but in some cases, the date issue can be resolved only by replacing the hardware.