12.1.2. 执行日期检查和时间计算

使用 DateTime::checked_add_signed 计算并显示两周之后的日期和时间，使用 DateTime::checked_sub_signed 计算并显示前一天的日期。如果无法计算出日期和时间，这些方法将返回 None。

可以在 chrono::format::strftime 中找到适用于 DateTime::format 的转义序列。

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

fn day_earlier(date_time: DateTime<Utc>) -> Option<DateTime<Utc>> {
    date_time.checked_sub_signed(Duration::days(1))
}

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

    let almost_three_weeks_from_now = now.checked_add_signed(Duration::weeks(2))
            .and_then(|in_2weeks| in_2weeks.checked_add_signed(Duration::weeks(1)))
            .and_then(day_earlier);

    match almost_three_weeks_from_now {
        Some(x) => println!("{}", x),
        None => eprintln!("Almost three weeks from now overflows!"),
    }

    match now.checked_add_signed(Duration::max_value()) {
        Some(x) => println!("{}", x),
        None => eprintln!("We can't use chrono to tell the time for the Solar System to complete more than one full orbit around the galactic center."),
    }
}