please allow do some thoughts outside - not really offtopic.
- your problem is a timing problem .. first of all there are simple to use features provided by operating system.
- you are talking about having the light on for 12 hours. why not set an alarm(43200) and catch this event?
your process then gets inactiv for a long time.
possible strategy eventdriven logic

{
  $state=off
  set_signal_handler()
  toggle_switch('time') #never get out of this function; see wait;

  set_signal_handler() {
    sig_alrm=toggle_switch()
    sig_int=switch_off_and_exit()
    sig_term=ignore
    sig_hup=switch_off_and_exit()
  }

  toggle_switch() {
     if ($stat==off) { $waitsec=43200 }
     else { $waitsec=calc_sec_to_switch('time') }
     alarm($waitsec)
     wait(-1) #go to sleep til alarm, or signals (INT,HUP)
     switch_on_off()
  }
}

have fun