const Add?

Define FIVE_SECONDS as const:

#![allow(unused)]
fn main() {
use std::time::Duration;
const fn unwrap<T: Copy>(o: Option<T>) -> T { match o { Some(t) => t, None => panic!() } }
const TWO_SECONDS: Duration = Duration::from_secs(2);
/*
const FIVE_SECONDS: Duration = TWO_SECONDS + Duration::from_secs(3);
*/
const FIVE_SECONDS: Duration = unwrap(TWO_SECONDS.checked_add(Duration::from_secs(3)));
assert_eq!(FIVE_SECONDS, Duration::from_secs(5));
}

Try solving it in the playground:

use std::time::Duration;

const TWO_SECONDS: Duration = Duration::from_secs(2);
const FIVE_SECONDS: Duration = TWO_SECONDS + Duration::from_secs(3);

fn main() {
    assert_eq!(FIVE_SECONDS, Duration::from_secs(5));
}