`Instant::PAST_VALUE` constant?

Yes, that's why I want this over calling that function. It would also work for me if it was allowed to be the past or the present. As for your concerns about people using it as a sentinel, I assume it would be fine so long as the docs come with a disclaimer that it's not to be used as a sentinel value, like NonNull::dangling() does. I haven't heard of any errors from someone mis-using that value as a sentinel.