Each database has a different format for Date, and that is what a dialect covers and what you put in for your mapping of that column, but I suggest looking at the hibernate documentation pages like in the faq here
http://www.hibernate.org/116.html#A37 And in the documentation the built-in types
http://www.hibernate.org/hib_docs/v3/reference/en/html_single/#mapping-types-basictypes I also highly recommend using bind parameters if you are going to use Dates in your where portion of your query.
http://www.hibernate.org/hib_docs/v3/reference/en/html_single/#objectstate-querying-executing-parameters SO what you didn't mention is how your query is created, is it a Criteria, Query or SQLQuery object your create from your session. And how do you build your query.
Mark