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";
            }

1、用【between *and】但要注意时间范围，例如：select * from tb where riqi between '2009-01-22 00:00:00' and '2009-01-22 23:59:59'
2、用【convert（）函数 】select * from tb where convert(varchar(10),riqi,120) = '2009-01-22'  

SQL拼接查询，要加上点如：'2016-1-12'