Shouldnt matter, I just created a table and put a CF on it and it highlighted.
You need to change A1 to whatever the top left cell is that you want to highlight.
So if your data is in B13:J46 you need to change the A1 cell to B13.
Maybe they are not dates, this is a common fallacy.
Choose a cell you think is a date, let's say A1
In a blank cell type
ISNUMBER(A1)
If it says FALSE then your dates are not dates, they are text and the CF prob won't work in which case you'll need to change them all to proper dates.