急，取两个日期之间的所有数据

//传日期差的参数
int   Days   =Convert.ToInt32(Request.QueryString[ "Days "].Trim());
//利用日期差的参数生成开始时间
string   StartTime=System.DateTime.Now.AddDays(-Days).ToString( "yyyy-MM-dd ");
//取系统当前为结束时间
string   EndTime=System.DateTime   .Now.ToString   ( "yyyy-MM-dd ");
//SQL语句
string   sqlstr   = "select   *   from   Article   where   PublishTime   BETWEEN   StartTime   AND   EndTime   order   by   PublishTime   desc ";

sql语句执行老是出错。

我检查时，发现开始日期和结束日期都是正确的。
以上sql语句如果改成
string   sqlstr   = "select   *   from   Article   where   PublishTime   BETWEEN   '2007-8-9 'AND   '2007-8-11 'order   by   PublishTime   desc ";
可以正确执行。

请各位大侠指教


------解决方案--------------------
string sqlstr = "select * from Article where PublishTime BETWEEN ' " + StartTime + " ' AND

' " + EndTime + " order by PublishTime desc "