Millis() and rollover tutorial

Tutorials
17

why 1000? so for all the negative numbers will be 1000? what would happen in the second, third…loop (all will be negative, so all will be 1000)? so it would not reach the interval time?

I want to run my code for 12 months, will it work?

#define publish_cycle 30000 
unsigned long lastPublish = 0;

int led = D7;
double reading = 0.0;
double mvolts = 0.0;
double tempC = 0.0;
double tempF = 0.0;
boolean flag = false;

double readingl=0.0;

void setup() {
    pinMode(led,OUTPUT);
    Particle.keepAlive(120);
    Particle.function("led", ledStatus);

}

void loop() {
    unsigned long now = millis();
    reading = analogRead(A0);
    mvolts = (reading * 3300.0)/4095.0;
    mvolts = mvolts -500.0;
    tempC = mvolts/10.0;
    tempF = (tempC*1.8) + 32.0;
    readingl = analogRead(A1);
    
   //to IFTTT
    if ( tempC > 26.0 && flag == false){
    Particle.publish("toohot");
    flag = true;
    }
    if (tempC < 24.0 && flag == true){
    Particle.publish("tempnormal");
    flag = false;
    }

    
    if ((unsigned long)(now - lastPublish) > publish_cycle) {
    //to LOSANT
    Particle.publish("temp-reading", String(tempC), PRIVATE) ;
    Particle.publish("light-intensity",String(readingl), PRIVATE) ;
    delay(500);
    //to ThingSpeak
    Particle.publish("temperature", "{ \"1\": \"" + String(tempC) + "\"," + "\"2\": \"" + String(readingl) + "\"}", PRIVATE);
    lastPublish=now;
    }
    

}





int ledStatus(String command){
    if(command.equalsIgnoreCase("on")){
        digitalWrite(led,HIGH);
        return 1;
    }
    else if (command.equalsIgnoreCase("off")){
        digitalWrite(led,LOW);
        return 0;
    }
    else{
        return -1;
    }
    
}