It only makes sense from a shortsighted "today's" perspective. What happens if there is another API change? Do you call the new new suffix _NEWER? _NEWNEW?
Thanksfully 64 is the endpoint. "We'll never need 128bit time_t struct's!" Famous last words. But really, we don't. We don't need to mix low precision with high precision times, "one for all" . Calculating in ns does not make sense in the year scale.