Convert problem with "date textmode" for textbox

protected void Grid_RowCommand(object sender, GridViewCommandEventArgs e)
        {

            if (e.CommandName == "ibutonsec")
            {
                int num = int.Parse(e.CommandArgument.ToString());

     
                txtboxStartDate.Text = gridViewIsemri.Rows[num].Cells[2].Text;


                txtboxEndDate.Text = gridViewIsemri.Rows[num].Cells[4].Text;
             }
 

            }

 Hi everyone 

I want to get StartTime and EndTime when i clicked blue button(You'll see above pic in gridview) . I can get time , text , numbers etc ... but i can't send value to date textbox from StartDateCell (IsEmriBasTar in pic) or EndDateCell .  How can i send value to date text box ? 

I tried by all possible ways many times  . Please advice me . Thank you very much .

Note: (text mode = date  for text box in .NET framework 4.5 )