Rust

But a day does have exactly 24 hours, at least when using Unix timestamps. Leap seconds aren't accounted for in a Unix timestamp, and Unix timestamps assume UTC timezone, which doesn't have daylight savings.

If you don't make a "from_days()" or "DAYS" constant, people will make their own and they'll be wrong in exactly the same way as if that function or constant was defined in the stdlib, but at least in the stdlib, you have the opportunity to centralize the documentation for it and maybe educate that junior dev before they make a mistake.