Using Find Control method you can find out the Text box that contains date.
Now assign this text box text property to specific object property.
TextBox txtdate= (TextBox)GridView1.Rows[e.RowIndex].FindControl("txtdate");
objBusinessUI.StockDate=txtdate.text;
or
objBusinessUI.StockDate=DateTime.Parse(txtdate.text);
if StockDate datatype is date.
Hope this one help you.