Thanks for the explanation. I agree signed spans make it easier to express concepts like "1 year ago" vs "1 year from now". And clearly if the concept of a negative duration has made it into the standard then it makes sense to support it. But I do wonder if there's some value still in being precise, if I think I've measured a negative duration--e.g I look at my watch and write down t0, wait a bit, look at my watch again and write down t1 and find t0 > t1--that's something surprising that probably warrants further investigation. The explanation likely isn't "well time just went backwards for a bit there" :P. This happens frequently in computers, though, so maybe making the programmer handle an error each time is excessive.
Yeah it's hard for me to get a sense of where that would really matter in practice, or even when it would be correct to do so. You could a `assert!(!span.is_negative());`. Although, you have to be careful, because system time can change arbitrarily.
Your watch example is actually an important use case that specifically isn't served by Jiff. You should be using monotonic time (in Rust, that's `std::time::Instant`) for things like that. In that case, you really do get a guarantee that the duration between two instants (one in the past and one in the future) is non-negative. And if it weren't, then that would be worthy of an assert. But you get no such guarantees with system time.
