使用以下方法更改日期格式。希望这有帮助

protected void Submit_click(object sender, EventArgs e)
{
    DateTime startdate = Convert.ToDateTime(txtstartdate.Text);
    DateTime enddate = Convert.ToDateTime(txtenddate.Text);
    for (DateTime date = startdate; date <= enddate; date = date.AddDays(1))
    {
        try
        {
            var shtdate = date.ToShortDateString();
            string MyConString = "SERVER=localhost;DATABASE=mydb;UID=myid;PASSWORD=abc123;";
            MySqlConnection connection = new MySqlConnection(MyConString);
            string cmdText = "INSERT INTO approved(agentlogin ,leavetype ,date ,time, reason)VALUES ( @login, @type, @date, 'Full day', @reason)";
            MySqlCommand cmd = new MySqlCommand(cmdText, connection);
            cmd.Parameters.AddWithValue("@login", Label1.Text);
            cmd.Parameters.AddWithValue("@type", ddlleavetype.Text);
            cmd.Parameters.AddWithValue("@date", shtdate);
            cmd.Parameters.AddWithValue("@reason", txtreason.Text);

            connection.Open();

            int result = cmd.ExecuteNonQuery();
            connection.Close();

            //lblError.Text = "Data Saved";

        }
        catch (Exception)
        {
            Console.Write("not entered");
            //lblError.Text = ex.Message;
        }
    }
}

这就是为什么你应该使用模型，并利用适当的类型，而不是保持一切一个简单的 string .
虽然不是十全十美，但朝着正确方向迈出的一步应该是这样的：（请记住，我并没有解决每一个问题。还有很大的改进空间）

public class DateRangeModel
{
    public DateTime From {get; set;}
    public DateTime To   {get; set;}

    public IEnumerable<DateTime> DaysInRange {
        get{
            for(DateTime date = StartDate; date.Date <= EndDate.Date; date = date.AddDays(1))
            {
                yield date;
            }
        }
    }

    public DateRangeModel(string from, string to)
    {
        From = GetDate(from);
        To = GetDate(to);
    }

    private static DateTime GetDate(string string_date)
    {
        DateTime dateValue;
        if (DateTime.TryParse(string_date, out dateValue))
            return dateValue;
        else
            throw new Exception("Unable to convert '{0}' to a date.", string_date);

    }
}

protected void Submit_click(object sender, EventArgs e)
{

    DateRangeModel dateRange = new DateRangeModel(txtstartdate.Text, txtenddate.Text);

    OleDbConnection scn = new OleDbConnection();
    scn.ConnectionString = @"Provider=Microsoft.Jet.OLEDB.4.0;Data   Source=/mysource;";

    foreach(var dt in dateRange.DaysInRange)
    {
        OleDbCommand scmd = new OleDbCommand("Insert query", scn);
        scmd.CommandType = CommandType.Text;
        scmd.Parameters.Clear();
        scmd.Parameters.AddWithValue("@date1", dt);
        scmd.Parameters.AddWithValue("reason", txtreason);
        //etc (finish your insert stmt here).
    }  
}