Some experimentation yields that the MDY function doesn't report an error (forward) until 1/1/20001. Other functions appear to work with dates until then as well.

I ran into this dealing with Y2K issues in 1999 and having to repeatedly respond that the dates I was using in SAS data sets were good. They shut up when I said the dates would work through the year 20,000.

And why would anyone be surprised with an Excel date conversion issue when Excel accepts 1/0/1900 as a "date". Try entering 1/1/1900 in a cell then a formula subtracting 1 (day) => 1/0/1900.