Interview with @jhpratt

Today, we are glad to announce our latest interview with Jacob Pratt, the core maintainer of the time crate and a contributor to the Rust compiler and standard library. We are grateful for Jacob’s time and expertise, and we look forward to sharing his insights with you and hope you enjoy our interview with Jacob Pratt.


Introduce yourself and share a bit about your background with Rust. When did you start learning Rust, and what inspired you to do so?


Hello! For those that don’t know me, I am Jacob Pratt (GitHub @jhpratt). I’ve been maintaining the time crate and contributing to rustc and std for a few years now. To the best of my recollection, I’ve been using Rust since October 2016, though I had played around with it somewhat before then. At that time, I was working on a backend for a website in JavaScript, but could not convince myself that a non-admin user would not be able to access the admin endpoints. This is because JavaScript is weakly typed, so any effort to verify this would have taken significant work. I had heard of this relatively new language “Rust”, and decided to check it out. As Rust is strongly typed, my concerns were immediately alleviated, as I could simply require an instance of an admin user as a parameter. As a nice bonus, the backend got significantly faster in the rewrite!

In the nearly seven years since I started using Rust, I have written an immense amount of code in it. A large part of this is maintenance of time and contributing to the Rust compiler and standard library.

time crate

How did you get involved with the time crate, and what has your experience been like as a core maintainer?

GitHub - time-rs/time: The most used Rust library for date and time handling.
The most used Rust library for date and time handling. - GitHub - time-rs/time: The most used Rust library for date and time handling.

Odd as it may seem for such a popular crate, I just asked. time was officially deprecated when I took it over, so the people with publishing permission and GitHub access were willing to hand it over once I had written sufficient code (so as to avoid malicious intent).

My experience as a maintainer has been largely positive. Of course it’s impossible to please everyone, but I believe that on the whole I have written a solid crate that is widely used without much issue.

What are the differences between chrono and time crate? Can you offer any advice to help users choose which crate to use for their specific needs?


Overall, time and chrono provide a generally similar API. For the best comparison, I honestly suggest that people just try them to see which one they like more. However, I’ll try to give a relatively simple overview.

time has three core types: Date, Time, and UtcOffset. These can be combined to form a PrimitiveDateTime (consisting of a Date and Time) or an OffsetDateTime (consisting of all three). chrono is similar in having NaiveDate, NaiveTime, FixedOffset, NaiveDateTime, and DateTime, respectively. A quick check shows that the sizes of all types are the same, with the exception of UtcOffset, which is three bytes to four for FixedOffset. With that said, time supports niche value optimization! So if you have Option<T>, your types won’t be any larger if at all possible. chrono does not do this.

Perhaps the largest difference between time and chrono is the choice of formatting. time prefers more powerful, readable descriptions (e.g. [weekday], [month repr:long] [day padding:none]), while chrono prefers a more limited description that is compatible with C (e.g. %A, %B %-d, %Y). I made this change out of frustration with the existing description, as it’s not clear what is actually being written. Even those familiar with the format must admit that it’s difficult to remember that %A corresponds to “Sunday” and %B “July”, for example. An unexpected advantage of the change is that I was able to implement many modifiers, such as case sensitivity. Format descriptions in time can also be constructed via a macro, which statically verifies the format and eliminates all runtime overhead of parsing it, leading to it being faster the last time I benchmarked it.

Has the time crate fixed the localtime soundness issue? If not, why? Have you considered using tz-rs to address the issue?


  • localtime soundness issue was reported by @quininer in 2020, both time and chrono have been affected. See time#294 and chrono#499 respectively.
  • tz-rs is an emerging crate reimplementation of libc functions localtime, gmtime and mktime.

“Fixed” is a relative term, unfortunately. It is fixed in the sense that there is no unsoundness (unless explicitly opted into via an unsafe method call), but the issue still remains in that Unix-based operating systems without thread-safe environments are unable to soundly get the local UTC offset. For that reason any method calls that require doing that return the error value.

With regard to tz-rs, I considered it when it was first released. Using that crate would be extremely costly, as it would require parsing the entire time zone database on every invocation to avoid returning incorrect values. For this reason I decided not to use it.

One solution that I hope will eventually be picked back up is #[deprecated_safe], which should be applied to std::env::set_var. Ultimately I would love for every operating system to have thread-safe environments, but that’s an issue dating back to the very introduction of environment variables in 1979.

Contribution to Rust

In addition to maintaining the time crate, what other contributions have you made to the Rust community? For example, I know you are the author of RFC and the implementation of derive_default_enum.


time is definitely the largest. Aside from that, I maintain deranged — a proof of concept for ranged integers — and num_threads — to get the number of threads the process is currently using. I previously maintained standback, which backports new features from the standard library to older compilers, but stopped maintaining it because no one used it.

As you mentioned, I also (somewhat) wrote the RFC for #[derive(Default)] on enums in addition to implementing and stabilizing it. More recently, I wrote the now-accepted RFC for restrictions, which will permit sealed traits at a compiler-level, rather than (ab)using visibility and accessibility. There is a pull request open to implement this, so it’ll hit nightly relatively soon. By the time this is live, it may already be on nightly!

Beyond the two RFCs, I’ve stabilized a large number of standard library items and language features, largely allowing new behavior in const contexts. Aside from const things, I implemented proper stability checks for impl const Trait, improved stability checks within std, and reduced the frequency of merge conflicts for PRs to the Rust repository. Something I’ve been looking into more recently is reducing the special-casing of the standard library, which will have significant benefits to the Rust ecosystem as a whole. I’ll actually be talking about this at RustConf in September!

What are the most rewarding part and biggest challenges that you have faced as a maintainer of time crate and contributor of rust?


Probably the most rewarding part is knowing that millions of people use code that I write on a daily basis. When I get a feature request, I know it’s because someone thinks the crate is useful and wants to see it improved, even if in a small way. The biggest challenge is unfortunately finding time to work on things. Funding is also far from ideal, but that’s the case for everyone. As far as technical challenges, I tend to push the boundaries of what’s possible with the compiler, leading me to occasionally run into barriers.


What is your vision for the future of the time crate and Rust as a whole?


I have a lot planned, to say the least.

In time, I started work on integrating the time zone database in April. That would mean full support for actual time zones, including daylight saving time adjustments, rather than only fixed UTC offsets. This is a large undertaking that will consume a significant amount of time to complete. Another thing I have been working on is having a generic DateTime type that supports a variety of offsets: statically known, dynamic (equivalent to OffsetDateTime), a time zone, or no offset at all (equivalent to PrimitiveDateTime). To that effect, both PrimitiveDateTime and OffsetDateTime are actually simple wrappers around this currently-internal type. I hope to eventually make it public, but there are some language features that I want to land on stable before that happens.

With Rust as a language, there are tons of things that I want to work on. I have an RFC that needs to be rewritten that would permit certain fields of structs/enums to have default values, such that they can be omitted when constructing them, with the values also being used by #[derive(Default)]. Other RFCs that I have planned, in some form or another, are unsafe fields and significant expansion of #[derive] capabilities, plus a couple more that I’m deliberately keeping the details of private. Rust is already a great language, but there is always room to improve. I intend to do my part in moving that forward.

End question

When you’re not working on Rust projects, what do you like to do in your free time? Do you have any hobbies or interests that you’re particularly passionate about?


While I do not train any more, I trained for nearly ten years in American Kenpo, specializing in self-defense and achieving the rank of third degree black belt. In that time, I taught others for a number of years. Another thing I do on occasion is solving Rubik’s cubes, though like the martial arts I don’t do this as much as I used to. Something I do quite often nowadays is watch various sports.

Jacob is in training for American Kenpo