That's all it needs to do; the actual starting point for the count is irrelevant so long as it counts the milliseconds elapsed accurately.
That depends on what you're using it for. You could argue that if Millisecs() consistently returned a unix timestamp on all platforms, it could be used for more purposes than simply measuring delta time.Ofcourse the really correct way to measure this is by using an accumulator, and incremental relative MilliSecs(). That way you can accurately measure time on as large a scale as you want, indefinately. Well until your accumulator overflows anyway.