how to calculate days between two dates using datatable and datarow in gridview ?

public void BindGrid(int rowcount)
   {
       btnsubmit.Visible = true;
       DataTable dt = new DataTable();
       DataRow dr;
       dt.Columns.Add(new System.Data.DataColumn("Employee ID", typeof(String)));
       dt.Columns.Add(new System.Data.DataColumn("Leave Type", typeof(String)));
       dt.Columns.Add(new System.Data.DataColumn("From", typeof(String)));
       dt.Columns.Add(new System.Data.DataColumn("To", typeof(String)));
       dt.Columns.Add(new System.Data.DataColumn("No:of Days", typeof(String)));


       if (ViewState["CurrentData"] != null)
       {
           for (int i = 0; i < rowcount + 1; i++)
           {
               dt = (DataTable)ViewState["CurrentData"];
               if (dt.Rows.Count > 0)
               {
                   dr = dt.NewRow();
                   dr[0] = dt.Rows[0][0].ToString();

               }
           }
           dr = dt.NewRow();
           dr[0] = lblempID.Text;
           dr[1] = ddlleavetype.SelectedItem.Text.ToString();
           dr[2] = txtfrom.Text.ToString();
           dr[3] = txtto.Text.ToString();

           dt.Rows.Add(dr);

       }
       else
       {
           dr = dt.NewRow();
           dr[0] = lblempID.Text;
           dr[1] = ddlleavetype.SelectedItem.Text.ToString();
           dr[2] = txtfrom.Text.ToString();
           dr[3] = txtto.Text.ToString();

           dt.Rows.Add(dr);

       }

       // If ViewState has a data then use the value as the DataSource
       if (ViewState["CurrentData"] != null)
       {
           gvLeave.DataSource = (DataTable)ViewState["CurrentData"];
           gvLeave.DataBind();
       }
       else
       {
           // Bind GridView with the initial data assocaited in the DataTable
           gvLeave.DataSource = dt;
           gvLeave.DataBind();

       }
       // Store the DataTable in ViewState to retain the values
       ViewState["CurrentData"] = dt;


   }