12/12 Led timer?

Simple example:

const unsigned long twelve_hours = 12*60*60*1000;
unsigned long elapsed;
bool light_state;

void loop()
{
  if ( millis() - elapsed >= twelve_hours )
  {
    light_state = !light_state;
    elapsed = millis();
            
    print( light_state ? "Light enabled" : "Light disabled" ); 
  }
}