Sometimes, set analysis does not handle the date format very well.
I suggest you use a auxiliary field for the DW date as an integer.
In the script, where you have something like:
, include another line
num(DW_LOAD_DATE) as nDW_DATE,
Your vConvStart and vConvEnd should get max and min of this new field
=min(nDW_DATE) and = max(nDW_DATE)
Your set analysis should also change to compare nDW_DATE instead of DW_LOAD_DATE
hope this helps,