You don't need any datetime arithmatic at all. Just loop endlessly and check once a minute wether the lamp state matches the daytime. 
use feature 'signatures';

my $on = 0;

sub switch_lamps ($switch_to) {
  state $lamp_state = 0;
  return if $lamp_state == $switch_to;
  $lamp_state = $switch_to;
  # ...
}

sub is_nighttime {
  #...
}

while (1) {
  switch_lamps( $on && is_nighttime() );
  sleep(60);
}

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holli

You can lead your users to water, but alas, you cannot drown them.

In reply to Re: Efficiency of indoor grow light timer by holli
in thread Efficiency of indoor grow light timer by stevieb

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