The general class of problems resulting from the wrapping of computers' internal date timers is given this label in honor of the most obvious occurrence -- when the year changes from 1999 to 2000 (abbreviated in some programs as 99 to 00 indicating a backwards time movement). Contrary to popular belief, these problems will not all manifest themselves on the first day of 2000, but will in fact happen over a range of dates extending out beyond 2075. A computer that does not have problems prior to the beginning of 2001 is considered "Y2K compliant", and a computer that does not have problems within the next ten years or so is considered for all practical purposes to be "Y2K clean". Whether or not a given computer is "clean" depends upon both its OS and its applications (and in some unfortunate cases, its hardware). The quick rundown on common home / small business machines (roughly from best to worst) is that:

• All Mac OS systems are okay until at least the year 2040. By that time a patch should be available.
• All BeOS systems are okay until the year 2040 (2038?). By that time a patch should be available.
• Most UNIX versions are either okay or currently have free fixes available (and typically would not have major problems until 2038 or later in any case).
• NewtonOS has a problem with the year 2010, but has a free fix available.
• Newer AmigaOS systems are okay; older ones have a problem with the year 2000 but have a free fix available. They also have a year 2077 problem that does not yet have a free fix.
• Some OS/2 systems have a year 2000 problem, but free fixes are available.
• All CP/M versions have a year 2000 problem, but free fixes are available.
• PC-DOS has a year 2000 problem, but a free fix is available.
• DR-DOS has a year 2000 problem, but a free fix is available.
• Different versions of GEOS have different problems ranging from minor year 2000 problems (with fixes in the works) to larger year 2080 problems (that do not have fixes yet). The only problem that may not have a fix in time is the year 2000 problem on the Apple ][ version of GEOS; not only was that version discontinued, unlike the other GEOS versions it no longer has a parent company to take care of it.
• All MS-Windows versions (except possibly Windows 2000 and Windows ME) have multiple problems with the year 2000 and/or 2001, most of which have free fixes but some of which still lack free fixes as of this writing. Even new machines off the shelf that are labelled "Y2K Compliant" usually are not unless additional software is purchased and installed. Basically WinNT and WinCE can be properly patched, Windows '98 can be patched to work properly through 2000 (possibly not 2001), Windows '95 can be at least partially patched for 2000 (but not 2001) but is not being guaranteed by Microsoft, and Windows 3.1 cannot be fully patched.
• MS-DOS has problems with at least the year 2000 (and probably more). None of its problems have been addressed as of this writing. Possible fixes are to change over to either PC-DOS or DR-DOS.

Results vary wildly for common applications, so it is better to be safe than sorry and check out the ones that you use. It should also be noted that some of the biggest expected Y2K problems will be at the two ends of the computer spectrum with older legacy mainframes (such as power some large banks) and some of the various tiny embedded computers (such as power most burgler alarms and many assorted appliances). Finally, it should also be mentioned that some older WinTel boxes and Amigas may have Y2K problems in their hardware requiring a card addition or replacement.