将传递的DateTime<FixedOffset>示例拆分为Date<FixedOffset>示例和NaiveTime示例，然后FixedOffset.ymd和.and_time使用传递的year创建新的DateTime<FixedOffset>示例。
Rust Playground

fn datetime_with_year(datetime: &DateTime<FixedOffset>, year: i32) -> DateTime<FixedOffset> {
    let date: Date<FixedOffset> = datetime.date();
    let time: NaiveTime = datetime.time();
    let fixedoffset: &FixedOffset = datetime.offset();
    match fixedoffset.ymd(year, date.month(), date.day()).and_time(time) {
        Some(datetime_) => {
            eprintln!("fixedoffset.ymd() Some {:?}", datetime_);
            datetime_
        }
        None => {
            eprintln!("fixedoffset.ymd() None");
            datetime.clone()
        }
    }
}

更新：或使用@Jmb推荐的datetime.with_year(year)。
啊！😑