You might try doing this in CurrentCellValidateString.
private void gridDataBoundGrid1_CurrentCellValidateString(object sender, GridCurrentCellValidateStringEventArgs e)
{
if(e.Text.IndexOf("t") > -1)
{
e.Cancel = true;
GridCurrentCell cc = this.gridDataBoundGrid1.CurrentCell;
cc.CancelEdit();
cc.Renderer.Control.Text = DateTime.Now.ToString();
this.gridDataBoundGrid1.RefreshRange(cc.RangeInfo, true);
}
}