it sounds like you just want the date portion, so if they are datetime fields, you could do something like:
day(parameter)=day(field) and month(parameter)=month(field) and year(parameter) = year(field)
there is bound to be an easier / more elegant method, but it just is popping into my head right now....this is the long way around to the answer.
you could proably strip the time off of the parameter by doing a dateAdd() and then seeing if your field is in the range from the parameter to parameter + 1 day...or 23:59:59 if you are paranoid.
HTH