Changing date backwards can also have negative impact on your system. For examples if you are logging historical data, you will be overwriting your data, if you are running a certain task weekly then by changing date backwards you will be altering that sequence too and so on and on....... By the way, how long you plan to keep changing date to a day before Year 2000 ? Do you have any embedded date sensitive circuitry in your system? For some system components, you may not be able to change the date at all. In my opinion, better way to deal with the YR 2000 issue is to test your system (in a safe environment) thoroughly. Find possible issues and fix them.