如何使用asp.net添加当前日期中的天数

我想将日期添加到当前日期，例如，我想用数字输入日期，然后选择要添加的日期。当前日期表示天、周和月，但当我选择天、周或月时，它会显示日期，但无法将其添加到当前日期有人能帮我打吗

 protected void TextBoxPredictDays_TextChanged(object sender, EventArgs e)
        {
            string selectPredictDays = DropDownList1.SelectedItem.Value;
            String Days1;
            int Result;
            Days1 = TextBoxPredictedClosing.ToString();
            if (selectPredictDays == "Days")
            {
                Result = Convert.ToInt32(TextBoxPredictDays.Text) * 1;
                TextBoxPredictedClosing.Text = Result.ToString();
            }
            else if (selectPredictDays == "Weeks")
            {
                Result = Convert.ToInt32(TextBoxPredictDays.Text) * 7;
                TextBoxPredictedClosing.Text = Result.ToString();
            }
            else if (selectPredictDays == "Months")
            {
                Result = Convert.ToInt32(TextBoxPredictDays.Text) * 30;
                TextBoxPredictedClosing.Text = Result.ToString();
            }
        }

这是我的设计代码

 <asp:TextBox ID="TextBoxPredictDays" runat="server" 
            ontextchanged="TextBoxPredictDays_TextChanged"  Width="101px" Height="14px" ></asp:TextBox>
      <asp:DropDownList ID="DropDownList1" runat="server" 
            onselectedindexchanged="DropDownList1_SelectedIndexChanged">
            <asp:ListItem>Days</asp:ListItem>
            <asp:ListItem>Weeks</asp:ListItem>
            <asp:ListItem>Months</asp:ListItem>
        </asp:DropDownList>

DateTime dt = DateTime.Now.AddDays(int); Days
dt = DateTime.Now.AddMonths(int);  Months
dt = DateTime.Now.AddDays(int * 7); Week

protected void TextBoxPredictDays_TextChanged(object sender, EventArgs e)
        {
            string selectPredictDays = DropDownList1.SelectedItem.Value;
            if (selectPredictDays == "Days")
            {
                TextBoxPredictedClosing.Text = DateTime.Now.AddDays(Convert.ToInt32(TextBoxPredictDays.Text));
            }
            else if (selectPredictDays == "Weeks")
            {
                TextBoxPredictedClosing.Text = DateTime.Now.AddDays(Convert.ToInt32(TextBoxPredictDays.Text) * 7);
            }
            else if (selectPredictDays == "Months")
            {
                TextBoxPredictedClosing.Text = DateTime.Now.AddMonths(Convert.ToInt32(TextBoxPredictDays.Text));
            }
        }