The first category will cause problems in hardware and software which assumes that the "00" means the year 1900, because the last two numbers are zero for both millenniums. Some hardware clocks calculate all dates based on a day count from a « day zero » baseline. Digital Equipment Corporation, Unisys, and Wang computer systems, IBM AS/400s, and some varieties of Unix will not recognize or register dates properly in the next millennium. After December 31, 1999, some versions of MS-DOS may reset the year to 1980 or 1984; and display the first day of the year 2000 as 01/01/00 but calculate a different date internally. Some software may divide by the last two digits and come up with divide-by-zero errors in the year 2000.

The second category will cause problems in software which uses the rule that a leap year is every year that can be exactly divided by 4, except for years which can be exactly divided by 100. The years 1700, 1800, and 1900 obey this rule, but 2000 does not. Software which must keep track of the days of each month (like archival backup software, calendars, groupware, spreadsheets) are specifically affected by this problem category.

The third category will cause problems when two date fields (a pre-2000 date and a post-1999 [erroneous] date) must be compared and an action taken based on the comparison. Examples of this include software or operating systems which check the creation dates of files with identical filenames and issue warnings or actions based on the comparison; keeping track of peoples' ages; keeping track of interest; and keeping track of billing and payment dates.

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