problem in date column with hibernate hql query

i hav a hql query..it gets the data used between two dates..i wriiten a query like this

from com.cod.model.Billing where datecolumn between '2011-4-4' and '2011-4-20'

but i didn't get any results from this query ..i checked out this query in mysql with sql query.there it's workng fine..but if i write same query in hql it returns null results can any one say what's the problem in my q开发者_开发百科uery.thanx in advance

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The dates mentioned have to be of the type java.util.Date and not String