In other words, there may be issues with displaying the date or problems with certain programs mis-reading the date. However, the operating system will continue to work.
First, Windows runs on top of DOS and may receive some information from DOS programs or programs using DOS functions. While MS-DOS 5.0 and higher (no earlier versions are being tested) are listed as compliant with acceptable deviation, there are a few issues relating to the date that might affect Windows. The Microsoft compliance statements for DOS are linked below.
Second, the BIOS of the computer may not be Year 2000 compliant. If this is the case, the computer may give the incorrect date. If the computer has the wrong date, DOS and Windows will have the wrong date.