Your DataContext is stale. Move the construction into the DoWork method: DataContext construction is very light anyway, so it will not harm performance that much.
private void backgroundWorker2_DoWork(object sender, DoWorkEventArgs e)
{
//==> Move it here clockEntities objDb = new clockEntities();
var inOutList = (from may in objDb.Taradods
where may.Date.Contains("2014/02") && may.BarCodeNo == 8011
select may);
this.Invoke(new MethodInvoker(delegate() {
dataGridView1.DataSource = inOutList.ToList(); }));
}