a special case of haiman's identity for the character of the quotient ring of diagonal coinvariants under the diagonal action of the symmetric group yields a formula for the bigraded hilbert series as a sum of rational functions in $q,t$. in this talk i will show how a summation identity of garsia and zabrocki for macdonald polynomial pieri coefficients can be used to transform haiman's formula for the hilbert series into an explicit polynomial in $q,t$ with integer coefficients. an equivalent formulation expresses the hilbert series as the constant term in a certain multivariate laurent series.