Formula not returning correct answer

I have a fomula that is returning the incorrect answer. The formula is as follows:

=IF(D2<DATEVALUE("10/1/2010"),"YES", "NO")

As I copy this formula down my column and the value in column D is equal to 10/1/10 or later (ie:7/1/11) the answer returned is is still YES instead of NO. Any suggestions on why it is doing this and how to fix?

thanks i'll give it a try