How do I convert a SystemTime to ISO 8601 in Rust?

Question

I have a SystemTime variable and I want to get the ISO 8601 format from that date.

score 19 · Accepted Answer · answered Oct 01 '20 at 02:16

19

The chrono package is the right tool for the job here. SystemTime may or may not be UTC, and chrono takes care of many irritating little details.

use chrono::prelude::{DateTime, Utc};

fn iso8601(st: &std::time::SystemTime) -> String {
    let dt: DateTime<Utc> = st.clone().into();
    format!("{}", dt.format("%+"))
    // formats like "2001-07-08T00:34:60.026490+09:30"
}

To customize the format differently, see the chrono::format::strftime docs.

answered Oct 01 '20 at 02:16

NovaDenizen

5,089
14
28

`std::time::SystemTime` is tagged with the `Clone` trait so passing a value instead of a reference should be preferred here. – Florian Dec 04 '22 at 08:20
Could you explain "SystemTime may or may not be UTC"? I don't see that mentioned in the docs and [this answer](https://stackoverflow.com/a/75714786/10238) to a similar question says "SystemTime itself is completely independent of timezones." – jmcnamara Jun 10 '23 at 00:09

score 13 · Answer 2 · answered Oct 01 '20 at 01:49

13

Convert it to a chrono::DateTime then use to_rfc3339:

use chrono::{DateTime, Utc}; // 0.4.15
use std::time::SystemTime;

fn main() {
    let now = SystemTime::now();
    let now: DateTime<Utc> = now.into();
    let now = now.to_rfc3339();

    println!("{}", now);
}

2020-10-01T01:47:12.746202562+00:00

The docs explain the naming choice of the methods:

ISO 8601 allows some freedom over the syntax and RFC 3339 exercises that freedom to rigidly define a fixed format

See also:

How to convert Unix time / time since the epoch to standard date and time?

answered Oct 01 '20 at 01:49

Shepmaster

388,571
95
1,107
1,366

1

You can also use the following .format() string to represent something close to JavaScript's .toISOString() function: `Utc::now().format("%Y-%m-%dT%H:%M:%S%.3fZ")` to get something like this: `2022-12-29T00:00:00.000Z` Playground Link: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=7cc9bcdc4022347b8a38bd81c84df3e9 – Vince Pike Dec 29 '22 at 14:09
1

in the latest version of chrono 0.4.23, it says here https://docs.rs/chrono/0.4.23/chrono/struct.DateTime.html#method.to_rfc3339 that its _"Available on crate features alloc or std only."_, which required me to add `features = ["alloc"]` to Cargo.toml like this: `chrono = { version = "0.4.23", default_features = false, features = ["alloc"] }` – Luke Schoen Mar 07 '23 at 01:41
but i got the time without SystemTime because i couldn't get it to work, i just used chrono with `let now: DateTime = Utc::now();` and added features `features = ["alloc", "clock"]` – Luke Schoen Mar 07 '23 at 02:03

score 4 · Answer 3 · answered May 22 '21 at 22:27

You can also use the time crate (doc). With the latest alpha release (0.3.0-ALPHA-0) you can use format_into() method while providing a &mut impl io::Write. Alternatively, you can simply use the format() method which is also compatible with older stable releases.

use time::{
    format_description::well_known::Rfc3339, // For 0.3.0-alpha-0
    // Format::Rfc3339, // For 0.2 stable versions
    OffsetDateTime
};
use std::time::SystemTime;

fn to_rfc3339<T>(dt: T) -> String where T: Into<OffsetDateTime> {
    dt.into().format(&Rfc3339)
}

fn main() {
   let now = SystemTime::now();
   println!("{}", to_rfc3339(now));
}

Playground

You will have to add the formatting feature to your Cargo.toml to use format_into() (i.e. formatting on v0.3+ require the feature to be enabled).

Note that you can also specify your own strftime-like format string to format/format_into.

How do I convert a SystemTime to ISO 8601 in Rust?

3 Answers3