The basic problem is that dates may be stored, or used for calculations, employing only the last 2 digits of the year. Erroneous results arise when calculating the duration from any 19xx year to any 20xx year.
Computers typically have 'realtime clock chips', that keep track of various timing functions.
Either may be designed or used without providing for the 1999-2000 transition, such that erroneous data are produced when the transition occurs. For example, Some PCs reset the system date to 1980 or other invalid dates when the computer reaches the year 2000. This problem is created by flaws in the computer hardware and in low-level BIOS software. The bad data then propagate through the system, leading to other errors in the functioning of the computer. Either type of software may have programming that do not provide for the 1999-2000 transition.