Reboot and try again. Has happened to me in the past and this usually fixes
it.
Just XL having one of its occasional hissy fits.

Failing this it may be bacause you have changed the location of the data
entry cell and the referenced to cells have locked '$' cell location values
in the formula and no longer relate to the cell with the original date. A
return of Jan 0 1900 or Jan 1 1900 usually means that you have no data in the
cell you are referencing to or the cell contains text. #VALUE would indicate
the same problem.