millis() returns an unsigned long integer, and C++ guarantees that
arithmetics on this type work modulo (ULONG_MAX+1). This behavior
ensures that computing a delay as
millis() - previousMillis
yields the correct result across millis() rollover events, as long as
the time actually elapsed is less than about 49.7 days.
Carter Nelson
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Bert Wijnants
Edgar Bonet
Matt Goodrich