var now = DateTime.Now;
var expirationDate = DateTime.Parse(row.Cells[2].Value.ToString());
var TwentyOneDays = expirationDate.AddDays(-21);

var TenDays = expirationDate.AddDays(-10);

if (now > TwentyOneDays && now < expirationDate)
{
     row.DefaultCellStyle.BackColor = Color.LightSkyBlue;
     row.Cells[2].Style.BackColor = Color.Yellow;

     // .....
}

DateTime expirationDate = DateTime.Now.AddDays(21);
DateTime now = DateTime.Now;

TimeSpan diff = expirationDate - now;

int days = diff.Days; // This would give 20

private void getexpireddate()
{
try
{
MySqlConnection con = new MySqlConnection(conn);
connect.Open();
string sql="SELECT expireddate FROM data1 where date(date) = date(now())";
MySqlDataAdapter da = new MySqlDataAdapter(sql, con);
DataTable ds = new DataTable();
da.Fill(ds);
datagrid.DataSource=ds;

}
catch(Exception ee)
{
Console.WriteLine(ee.Message);
}
}