get system uptime in rust

main.rs
use std::fs::File;
use std::io::Read;
use std::time::Duration;
use chrono::prelude::*;

fn get_uptime() -> Duration {
    let mut uptime_str = String::new();
    let mut f = File::open("/proc/uptime").expect("Failed to open /proc/uptime");
    f.read_to_string(&mut uptime_str).expect("Failed to read /proc/uptime");

    let uptime_secs = uptime_str.split('.').next().expect("Malformed /proc/uptime").parse::<u64>().expect("Malformed /proc/uptime");
    Duration::from_secs(uptime_secs)
}

fn main() {
    let uptime_duration = get_uptime();
    let datetime = Utc::now() - uptime_duration;
    println!("System started at {}", datetime.to_rfc2822());
}
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