"Also, as you can hold onto the timer, make sure invoking start/'end' multiple times in arbitrary order gives meaningful results."

I do not understand. Is AssertionError: thread already started a meaningful result? That is what Timer raises if you call start() on it multiple times. The intent of PeriodicTimer is to not be reusable because I have not seen any use case for that, but I guess I could make it so if you think so.