I've run into something similar a long time ago.

I found that in some cases (usually in really old databases that have been around since DF2.3) I might have a date that literally only has 2 digits for the year. I had trouble spotting it because everytime I was looking at the date in a DF program or utility it automatically showed the year as 4 digits. I think I spotted it by outputting the record using Sture's dfquery. Once I knew what field was the problem, I simply retyped it in in the explorer to fix the problem.