(1) An instrument is payable at a definite time if by its terms it is payable (a) On or before a stated date or at a fixed period after a stated date; or (b) At a fixed period after sight; or (c) At a definite time subject to any acceleration; or (d) At a definite time subject to extension at the option of the holder or to extension to a further definite time at the option of the maker or acceptor or automatically upon or after a specified act or event. (2) An instrument which by its terms is otherwise payable only upon an act or event uncertain as to time of occurrence is not payable at a definite time even though the act or event has occurred.