Escrevi o código assim:
void loop() {
if (condition1){
unsigned long current_millis = millis();
if (current_millis - previous_millis > interval){
previous_millis = current_millis;
if (condition2){
unsigned long current_millis = millis();
if (current_millis - previous_millis > interval){
previous_millis = current_millis;
if (condition3){
lcd.clear();
lcd.setCursor(0, 1);
lcd.print("tempo: ");
unsigned long tempo = millis();
lcd.println(tempo/1000);
}
}
}
}
}
}