Looking for guidance in frequency measurement

Projects General Guidance
20

Tone may be suitable for this. I started to write my own very basic version, however for some reason my millis function isn't working now. I thought it was a problem with the timers, but I'm not using them now so I'm pretty confused as to why this is happening. My understanding is that millis is a built in method that should work out of the box? If the below works for you, I wonder if my Arduino is damaged?

unsigned long currentTimeStamp;
unsigned long serialLastWriteTimeStamp;

void setup() 
{
    Serial.begin(9600);
    while (!Serial)
    {
      ; // Wait for serial to connect
    }

    currentTimeStamp = millis();
    serialLastWriteTimeStamp = millis();

    Serial.print("currentTimeStamp: "); Serial.println(currentTimeStamp);
    Serial.print("serialLastWriteTimeStamp: "); Serial.println(serialLastWriteTimeStamp);
    Serial.print("currentTimeStamp - serialLastWriteTimeStamp: "); Serial.println(currentTimeStamp - serialLastWriteTimeStamp);

}

void loop() {
  // put your main code here, to run repeatedly:

}