What exactly do you mean when you say "he calendar controll still remain on the screen."?
I can't see any reason why the Update method would work any differently on an unbound cell or in a DateTime cell than in any other cell. But Update might not work if the cell is still in the middle of an edit. You might have to call grid.PerformAction(ExitEditMode) before you call Update on the row.
ok with grid.PerformAction(ExitEditMode) work fine,
thanks mike