How do i make a timed switch?

I need to create a timed switch (about 8 hours long) that will turn on when the user presses a button and turn off 8 hours later. i don't know voltages... so i just need possible parts. its for a boiler, so it will probably be 12-120 volts.
Update: Can i get the IDEC RTE at lowes/home depot? or do i need to order it online? thanks!
Update 2: The control circuit is 120v. just adding details as i find them.
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