How to Calculate Date difference from two textboxes value on selection of date from Datepicker of second textbox in GridView row using C# and VB.Net in ASP.Net

Hi

I have created a sample which full fill your requriement you need to modify the code according to your need

HTML

<div>
    <asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="false" AutoGenerateEditButton="true"
        OnRowCancelingEdit="GridView1_RowCancelingEdit" OnRowEditing="GridView1_RowEditing"
        OnRowUpdating="GridView1_RowUpdating">
        <Columns>
            <asp:TemplateField HeaderText="Start Date">
                <ItemTemplate>
                    <asp:Label ID="lblStartDate" Text='<%# Eval("StartDate") %>' runat="server" />
                </ItemTemplate>
                <EditItemTemplate>
                    <asp:TextBox ID="txtStartDate" Text='<%# Eval("StartDate") %>' runat="server"></asp:TextBox>
                </EditItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="End Date">
                <ItemTemplate>
                    <asp:Label ID="lblEndDate" Text='<%# Eval("EndDate") %>' runat="server" />
                </ItemTemplate>
                <EditItemTemplate>
                    <asp:TextBox ID="txtEndDate" Text='<%# Eval("EndDate") %>' AutoPostBack="true" OnTextChanged="TimeSpan"
                        runat="server"></asp:TextBox>
                </EditItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Difference Time">
                <ItemTemplate>
                    <asp:Label ID="lblTimeDifference" runat="server" />
                </ItemTemplate>
                <EditItemTemplate>
                    <asp:TextBox ID="txtTimeDifference" runat="server"></asp:TextBox>
                    <asp:HiddenField ID="hfTimeDifference" runat="server" />
                </EditItemTemplate>
            </asp:TemplateField>
        </Columns>
    </asp:GridView>
</div>
<div>
    <link rel="stylesheet" href="//code.jquery.com/ui/1.12.1/themes/base/jquery-ui.css" />
    <link rel="stylesheet" href="/resources/demos/style.css" />
    <script type="text/javascript" src="https://code.jquery.com/jquery-1.12.4.js"></script>
    <script type="text/javascript" src="https://code.jquery.com/ui/1.12.1/jquery-ui.js"></script>
    <script type="text/javascript">
        $(function () {
            $("[id*=txtStartDate]").datepicker({ showSecond: true, dateFormat: 'mm/dd/yy' });
            $("[id*=txtEndDate]").datepicker({ showSecond: true, dateFormat: 'mm/dd/yy' });
        });
    </script>
</div>

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        BindGrid();
    }
}

private void BindGrid()
{
    DataTable dt = new DataTable();
    dt.Columns.AddRange(new DataColumn[3] { new DataColumn("StartDate"), new DataColumn("EndDate"), new DataColumn("Country") });
    dt.Rows.Add(DateTime.Now.AddDays(-35).ToShortDateString(), DateTime.Now.AddDays(10).ToShortDateString());
    dt.Rows.Add(DateTime.Now.AddDays(-15).ToShortDateString(), DateTime.Now.AddDays(15).ToShortDateString());
    dt.Rows.Add(DateTime.Now.AddDays(-45).ToShortDateString(), DateTime.Now.AddDays(3).ToShortDateString());
    dt.Rows.Add(DateTime.Now.AddDays(-12).ToShortDateString(), DateTime.Now.AddDays(36).ToShortDateString());
    GridView1.DataSource = dt;
    GridView1.DataBind();
}

protected void GridView1_RowEditing(object sender, GridViewEditEventArgs e)
{
    GridView1.EditIndex = e.NewEditIndex;
    BindGrid();
}

protected void GridView1_RowUpdating(object sender, GridViewUpdateEventArgs e)
{
    GridView1.EditIndex = -1;
    BindGrid();
}

protected void GridView1_RowCancelingEdit(object sender, GridViewCancelEditEventArgs e)
{
    GridView1.EditIndex = -1;
    BindGrid();
}

protected void TimeSpan(object sender, EventArgs e)
{
    GridViewRow row = (sender as TextBox).NamingContainer as GridViewRow;
    string txtStartDate = (row.FindControl("txtStartDate") as TextBox).Text;
    string txtEndDate = (row.FindControl("txtEndDate") as TextBox).Text;
    TimeSpan timeSpan = Convert.ToDateTime(txtEndDate) - Convert.ToDateTime(txtStartDate);
    TextBox txtTimeDifference = (row.FindControl("txtTimeDifference") as TextBox);
    txtTimeDifference.Text = timeSpan.ToString();
}

VB.Net

Protected Sub Page_Load(sender As Object, e As EventArgs) Handles Me.Load 
	If Not Me.IsPostBack Then
		BindGrid()
	End If
End Sub

Private Sub BindGrid()
	Dim dt As New DataTable()
	dt.Columns.AddRange(New DataColumn(2) {New DataColumn("StartDate"), New DataColumn("EndDate"), New DataColumn("Country")})
	dt.Rows.Add(DateTime.Now.AddDays(-35).ToShortDateString(), DateTime.Now.AddDays(10).ToShortDateString())
	dt.Rows.Add(DateTime.Now.AddDays(-15).ToShortDateString(), DateTime.Now.AddDays(15).ToShortDateString())
	dt.Rows.Add(DateTime.Now.AddDays(-45).ToShortDateString(), DateTime.Now.AddDays(3).ToShortDateString())
	dt.Rows.Add(DateTime.Now.AddDays(-12).ToShortDateString(), DateTime.Now.AddDays(36).ToShortDateString())
	GridView1.DataSource = dt
	GridView1.DataBind()
End Sub

Protected Sub GridView1_RowEditing(sender As Object, e As GridViewEditEventArgs)
	GridView1.EditIndex = e.NewEditIndex
	BindGrid()
End Sub

Protected Sub GridView1_RowUpdating(sender As Object, e As GridViewUpdateEventArgs)
	GridView1.EditIndex = -1
	BindGrid()
End Sub

Protected Sub GridView1_RowCancelingEdit(sender As Object, e As GridViewCancelEditEventArgs)
	GridView1.EditIndex = -1
	BindGrid()
End Sub

Protected Sub TimeSpan(sender As Object, e As EventArgs)
	Dim row As GridViewRow = TryCast(TryCast(sender, TextBox).NamingContainer, GridViewRow)
	Dim txtStartDate As String = TryCast(row.FindControl("txtStartDate"), TextBox).Text
	Dim txtEndDate As String = TryCast(row.FindControl("txtEndDate"), TextBox).Text
	Dim timeSpan__1 As TimeSpan = Convert.ToDateTime(txtEndDate) - Convert.ToDateTime(txtStartDate)
	Dim txtTimeDifference As TextBox = TryCast(row.FindControl("txtTimeDifference"), TextBox)
	txtTimeDifference.Text = timeSpan__1.ToString()
End Sub

ScreenShot