When a PC starts up, DOS gets the date and time from the BIOS, which in turn gets the date and time from the RTC. DOS has a 4-digit representation of the year, which it assembles from the year in the RTC and the century byte from the CMOS settings.

When the machine is powered on, both DOS and the RTC are keeping track of the date and time independently of each other and DOS will roll over from 31/12/1999 to 01/01/2000. However, DOS is not updating the date and time of the RTC and will not do so unless the DOS or Windows DATE or TIME commands are used.

So if the PC is switched off after the turn of the century, the next time it is powered up, DOS will get the year from the RTC - which will say 00 – and the century from the CMOS – which will still say 19! However, 1900 is not valid date for most PCs (must be between 1980 and 2099) so the date will be set to 01/04/1980, which is the DOS default for when an out-of-range date is found.

Most major manufacturers have issued statements about whether or not their PCs have problems, but testing is also reasonably straightforward. The easiest way to find out in whether your PC has a problem is to test it and see what happens when you:

• let it roll over the century, power it off and then on again;
• power it off just before 2000 and power it on again after the century change; and
• reset the DATE in 2000 power it off and power it on again.

We must inject here a note of caution, as we have read examples where a PC re-booting to 01/04/1980 has caused problems with file dates etc. We therefore recommend that you use a spare PC, or boot up your PC from a diskette, or at least perform a full back-up before carrying out your set of tests. There is a detailed description of how to boot up safely in web site reference in Appendix E item 3.

Where PCs have a problem, it can be solved in a variety of simple ways:

• a BIOS fix, which effectively looks at the year on the RTC clock and makes the assumption that if the year is less than 80, the century byte should be set to 20 (may be available from the vendor or one of a range of freeware available from the Web);
• a procedured fix: use the DOS DATE command to reset the date to the new century before powering the PC down;
• a nuisance fix. We have heard of PCs where the CMOS century byte refuses to be reset. This means that every time the PC is restarted, the DATE command needs to be used to properly set the DOS century. Insert the DATE command in the AUTOEXEC.BAT file. This is a nuisance because the user has to enter the date at a prompt each time the PC is fired up. If you have PCs with this problem then get on to the supplier to provide a more elegant fix;
• a replacement fix: this sounds a bit drastic, but many PCs will be replaced through natural evolution before the year 2000 arrives. Priority could be given to those that have problems – but make sure that any replacements do not suffer from the same error!