Where is LED_BUILTIN?
don't know, it mus be defined somewhere or the compiler would have complained about it, on the DUE it is on pin 13, maybe you can also define something as pin 13 yourself and use that.
Function millis() only need be called once.
yes but now you've declared an extra 4 byte variable, it all depends what is your most precious resource (atm none i suppose) The other changes you made can be down to style, but
};
};
these semicolons really shouldn't be there(they don't actually do anything), in fact a closing brace is preferred on a line by itself except within the declaration of a class, then it can be followed by a semicolon.
Do you program in Java ?