Comment by sushibowl

Overall this looks nice, but I found myself stumbling over the ToSpan syntax:

    let span = 5.days().hours(8).minutes(1);

It feels sort of weird how the first number appears in front, and then all the other ones are function arguments. I suppose if you don't like that you can just write:

    let span = Span::new().days(5).hours(8).minutes(1);

at the expense of a couple characters, which is not too bad.