This really puzzled for hours, I searched all over the internet, but got no working solution. Can someone point where the problem is ... thanks ! I created my own dialect class

public class MySQLDialectExtended : MySQLDialect
{
    public MySQLDialectExtended()
    {
        RegisterFunction("date_add_interval", new SQLFunctionTemplate(NHibernateUtil.Date, "date_add(?1, INTERVAL ?2 ?3)"));            
    }
}

Then I try to use it as follows:

query.Append(
   " ( date_add_interval(D.ApprovalDate, 1, YEAR) < current_timestamp() <  date_add_interval(D.RenewalDate, -1, YEAR) )");

It fails with following exception:

NHibernate.Hql.Ast.ANTLR.QuerySyntaxException : Exception of type 'Antlr.Runtime.NoViableAltException' was thrown. near line 1, column 677

where the column number is at the end of the first 'YEAR' word.

Edit: here is my configuration

    <property name="dialect">MyCompanyName.MySQLDialectExtended, MyCompanyName</property>
    <property name="hbm2ddl.keywords">none</property>