Abstract: An electrical timer switch for determining the times of day at which electricity is to be supplied to a load has different seasonal timing schedules and includes a selector circuit which selects the correct schedule in accordance with the season of the year. The timer switch may be mounted between the contact elements of an adapter which is arranged to be located between a supply of mains electricity and an electricity meter. Initial time and calendar information are supplied to the time switch by a separate monitor unit comprising a battery-fed set-up box.