I have been (still am) batteling the same kind of problem. So far the best work-around I have found is to use the calander class to provide the parameter. I have another form where the calander is inappropriate and run into the 'ambiguios date' error. I re-coded that to format the date into strict date format, basicly, I check the format of the data entered, it could be text or date, if numeric, it is a date, then i take it apart(year_part, month_part, day_part) and put it back together with DATE(year_part,month_part, day_part)
Ugly, ungainly but it works, just so long as the users always use dd/mm/yyyy (offical date format)or else there will be no records found (day < 12) or invalid field (day >12)