If it's just about a single value you need to check and transform, I would use a conditional statement (i.e. if() statement ) in your load:
if( DATEFIELD = makedate(2012,6,30), makedate(2012,7,1), DATEFIELD) as DATEFIELD,
Your DATEFIELD should already be interpreted as date by QV, if not (i.e. still pure text, no numerical representation), use the date#() function to interpret your DATEFIELD as date.
Hope this helps,