Fix timestamp subtraction

[gadomski]

I'm pretty sure this will panic because you can't add calendar units (days or greater) to a Timestamp. It looks like you probably are looking to add 86400s - 1ns right? If so, I'd suggest:

start + jiff::SignedDuration::new(86_400, -1)

Originally posted by @BurntSushi in 3e980e3

[bitner]

Fixed in #67