Abstract: A calendar and to-do list software system, method, and user interface for operating an electronic digital computer is disclosed. Information regarding past recurring and future events, appointments, and tasks is stored in conjunction with information relating to the optimally desired frequency of the events, appointments, and tasks as well as information relating to the required amount of preparation time for future events, appointments, and tasks. Events may be displayed to the user in the form of one or more lists, for example two lists presented in side-by-side columns, one for past recurring events, appointments, and tasks, and the other for future events, appointments, and tasks. Events, appointments, and tasks may be sorted in order of a calculated priority, thereby assisting the user in determining what he or she should optimally do at the time of viewing of the calendar.