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

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.

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?

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.