For example, the Federal Government rolls over to FY2000 in October 1999. For businesses, governments and any entity that uses accounting software, the Fiscal Year 2000 may appear as FY1900 if the year is stored as a two-digit quantity and the century is assumed to be 1900. This is dealt with in detail by many Y2K sites around the web. Do not use 2-digit years in any data, or in any output that could conceivably be used as input by another program, local or remote. Just to be safe, you should even use 4-digit years when printing them for user display.
Many programs compute their own dates from internal data structures.
Cases have been found where they fail to identify 2000 as a leap year, because years divisible by 100 are not leap years unless they're also divisible by 400. 1800 and 1900 were not leap years but 2000 will be. Most Unix systems will handle this correctly and it's easy to test by setting a test system's date to the Leap Day, February 29, 2000. Systems which fail to recognize this will be a day off on all days past the Leap Day in 2000. The Y2K FAQ indicates that Windows 95 and NT operating system clocks can handle up to 2099 but some applications may use dates after 2000 or 2020 to turn the system clock back to 1900 or 1920, respectively. The 2.1 billion seconds covered by this algorithm will run out in the year 2038.