Help with Calculating the date

<asp:TemplateColumn HeaderText="Start Date">
						    <ItemTemplate>
					        <peterCC:DateTextBox ID="txtDateStart" runat="server" xEnableContextMenuB="False" Width="75px" >
                                    <xPopupCalendar xToggleImageUrl="PetersDatePackage/Appearance/Calendar.jpg" > </xPopupCalendar>
                            </peterCC:DateTextBox>
                       	    </ItemTemplate>
                            <ItemStyle Width="75px" />
                            <HeaderStyle Width="75px" />
					    </asp:TemplateColumn>
					    <asp:TemplateColumn HeaderText="# Weeks">
						    <ItemTemplate>
						        <asp:TextBox ID="txNoWeeks" runat="server" Text="" Width="30px" MaxLength="4" AutoPostBack="true" 
						        OnTextChanged="TextBox_Changed"  ></asp:TextBox>
					   	    </ItemTemplate>
                            <ItemStyle Width="75px" />
                            <HeaderStyle Width="75px" />
					    </asp:TemplateColumn>
					    <asp:TemplateColumn HeaderText="End Date">
						    <ItemTemplate>
					        <peterCC:DateTextBox ID="txtDateEnd" runat="server" xEnableContextMenuB="False" Width="75px" >
                                    <xPopupCalendar xToggleImageUrl="PetersDatePackage/Appearance/calendar.jpg" > </xPopupCalendar>
                            </peterCC:DateTextBox>
                    	    </ItemTemplate>
                            <ItemStyle Width="75px" />
                            <HeaderStyle Width="75px" />
					    </asp:TemplateColumn>
 
 
 
 
 
 
In my code behind I am doing the following:
 
protected void TextBox_Changed(object sender, EventArgs e)
    {
        int intWeeks = 0;
         
        foreach (DataGridItem item in dgPhaseDisplay.Items)
        {
            TextBox txtWeeks = (TextBox)item.FindControl("txNoWeeks");
            DateTime txtEndDate = (DateTime)item.FindControl("txtDateEnd");
            DateTime txtStartDate = (DateTime)item.FindControl("txtDateStart");
 
            if (intWeeks < 0)
            {
                SetValidationErrorMessage("Please enter the numeric value in the number of weeks.");
            }
            else
            {
                int intNumDays = 0;
                intNumDays = Convert.ToInt32(txtWeeks) * 7;
                txtEndDate = Convert.ToDateTime(txtStartDate).AddDays(14).ToString("d"); 
 
            }
        }    
         
    }