Error The conversion of a varchar data type to a datetime data type resulted in an out-of-range value

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C#

ASP.NET

SQL-Server

hye, i'm having a problem when i'm inserting data in textbox.the problem is when i'm searching for data 10/10/2010, it work perfectly but when i try to search other date (eg 25/11/2013), i would get error The conversion of a varchar data type to a datetime data type resulted in an out-of-range value. if i change the date to 11/22/2013, i will get error, String was not recognized as a valid DateTime. is there something wrong with my code? i will put where the error occur below.

protected void searchdata_Click(object sender, EventArgs e)
{
    Label1.Text = locID.ToString();
    adddate();
    byday2();

}

public void adddate()
{
    today = DateTime.Parse(txtStartDate.Text);//String was not recognized as a valid DateTime
    DateTime answer = today.AddDays(1);
    adddate2 = answer;
}
public void byday2()
{
    Label9.Text = adddate2.ToShortDateString();
    Label10.Text = txtStartDate.Text;
    if (Byday.Checked == true)
    {
        if (Maxdata.Checked == true)
        {
            //tablemax.hidden = true;
            lblmaxdata.Visible = true;
            lblmaxdata.Text = "Highest Data";
            
            string connectionString = "Data Source=.\\SQLEXPRESS;AttachDbFilename=C:\\Users\\shafiq\\Desktop\\cuba\\App_Data\\radiation.mdf;Integrated Security=True;User Instance=True";

            // Create SQLDataSource.
            SqlDataSource sqlDataSource = new SqlDataSource();
            sqlDataSource.ID = "SqlDataSource123";
            this.Page.Controls.Add(sqlDataSource);

            // Bind ConnectionString to SQLDataSource.
            sqlDataSource.ConnectionString = connectionString;
            sqlDataSource.SelectCommand = "SELECT top 3 [date], [data] FROM [loc1] WHERE (([data] >= '2') AND ([date] >= '" + today + "') AND ([date] < '" + adddate2 + "')) ORDER BY [data] DESC, [date] DESC";

            // Bind SQLDataSource to GridView after retrieving the records.
            gridmaxdata.DataSource = sqlDataSource;
            gridmaxdata.DataBind(); //The conversion of a varchar data type to a datetime data type resulted in an out-of-range value.
        }
    }
}