Is the std feature useful?

[pitdicker]

The std feature controls only one thing: the error returned by Tz::from_str implements std::error::Error.
That is not useless, it is just such a small thing we wouldn't implement a feature for it otherwise.

[djc]

I think both (a) being able to work in no_std contexts and (b) implementing std::error::Error for error types are valuable, and having a std feature doesn't seem like a big drag? So my answer would be yes.