Not everyone uses the same terminology when discussing Year 2000 issues. Others use the terms to indicate the degree of manual intervention required. This may require extensive re-editing of existing data. Moreover, odd things happen when arithmetic is performed on such data. Sorting is also affected.
Some programs use special dates as so-called sentinels.
Sentinel values are meant to never occur as actual data. In 1960, this may have seemed a good way to pack extra information into the date entry, but in 1999 it is clearly a problem. The year 2000 is a leap year. Hence 1800 and 1900 were not leap years, but 2000 will be. Everyone knows the first rule, and many know the second, but the third rule, being the least frequently applied, is least known.