Abstract: Scheduling circuitry, for use for example in an ATM network unit to schedule cell transmissions, includes a master calendar (1) for holding entries corresponding respectively to events (cell transmissions) that are to occur within a preselected master-calendar scheduling range (SR), and a slave calendar (12) for holding entries corresponding respectively to events that are to occur beyond that scheduling range. When an event is to be scheduled, calendar control circuitry (24) makes an entry corresponding thereto in the slave calendar (12) if the interval between a current time and a desired scheduling time for the event exceeds said scheduling range. The entry in the slave calendar includes timing information representing the desired scheduling time. The calendar control circuitry monitors the entries in the slave calendar (12) and causes an entry therein whose corresponding event becomes within the scheduling range to be transferred to the master calendar (1).