关于c#日期查询问题

string time1 = rkrq1.Value.ToShortDateString();
string time2 = rkrq2.Value.ToShortDateString();
           
            if (time1.Trim() != "")
            {
                cond = cond + "and 日期>=" + time1;
            }
            if (time2.Trim() != "")
            {
                cond = cond + "and 日期>=" + time2;
            }
if (cond != "")
            {
                sql = "select * from rkb where 1=1  " + cond;
            }
            else
            {
                sql = "select * from rkb";
            }