Looking for guidance in frequency measurement

Projects General Guidance
21

Seems like it does just what I'd expect:

currentTimeStamp: 0
serialLastWriteTimeStamp: 0
currentTimeStamp - serialLastWriteTimeStamp: 0

Do you not get zeros?

These happen almost intantaneously and read the same millis() millisecond:

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

I'd swap their order and put a delay in between to see something other than zeros.