#define period 3000

void timerRun(){
   static unsigned long time_now = 0;
   if(millis() - time_now > period){
        time_now = millis();
        Serial.println("Hello");
    }
}  
void setup() {
    Serial.begin(115200);
    //unsigned long range is 0 to 4,294,967,295 (2^32 - 1)
    unsigned long a = 1;
    unsigned long b = 4294967295; //unsigned long maximum value
 
    Serial.println(a-b);  //2   
}
  
void loop() {
 timerRun();
}