Electric Time Switch
NO= normally open
NC= normally closed
COM= common (to the associated NO and NC contacts)
you need to run a 120 volt circuit for the clock and attach the hot to terminal 1 and the neutral to terminal 2.
then, for your 240 volt circuit,
run one of the circuit conductors to the COM terminal. The other circuit conductor will pass straight through without any connections to the timer. Then, along with that passed through circuit conductor, attach a wire to the NO terminal and take both of them to the heater or whatever you are operating.
there is a diagram in your owners manual.