How do I add days to a Chrono UTC?

Just use Duration and appropriate operator:

use chrono::{Duration, Utc};

fn main() {
    let dt = Utc::now() + Duration::days(137);

    println!("today date + 137 days {}", dt);
}

Test on playground.

I just wanted to improve on @Stargateur answer. There is no need to use time crate, since chrono crate has Duration struct in it:

extern crate chrono;

use chrono::{Duration, Utc};

fn main() {
    let dt = Utc::now() + Duration::days(137);

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

Another test on playground

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