In a computer programming language environment wherein operations are specified in one or more parallel frame sequences of frames arranged on a time scale, a programming language provides user-specifiable semantics for defining the start times for starting the frames and the end times for ending the frames. A method may be executed according to a routine constructed using the programming language, wherein the method includes providing user-specified semantics for defining the start times for starting the frames and for defining the end times for ending the frames, and executing operations according to the start times and end times for the frames as defined by the user-specified semantics.