First, you don't need that structure to get the selected value - as long as a value has been selected, you can just use =DATA_DATE. If no value has been selected this won't work, but neither will your formula (you can't date() multiple values).
Second, I believe this should get you the last date that's smaller than the selected one, providing a selection has been made:
max({<DATA_DATE= {"<$(=[DATA_DATE])"} >} DATA_DATE)
Note that this assumes your values are already valid dates, as this won't work correctly on text strings.
