Converting from NaiveDate to DateTime<Tz> is currently hard

[Ekleog]

This issue is for documentation, and would be fixed by #883.

It is currently hard to convert from NaiveDate to DateTime, due to timezone shifts possibly occurring. The following code seems reasonable:

fn midnight_on(date: chrono::NaiveDate, tz: chrono_tz::Tz) -> chrono::DateTime<chrono_tz::Tz> {
    let datetime = date.and_hms_opt(0, 0, 0).unwrap();
    datetime.and_local_timezone(tz).unwrap()
}

but fails when passed for instance:

fn main() {
    let date = chrono::NaiveDate::from_ymd_opt(2064, 11, 2).unwrap();
    let tz = chrono_tz::Tz::America__Havana;
    println!("{:?}", midnight_on(date, tz));
}

with the error

thread 'main' panicked at 'Ambiguous local time, ranging from 2064-11-02T00:00:00CDT to 2064-11-02T00:00:00CST', /home/ekleog/.cargo/registry/src/github.com-1ecc6299db9ec823/chrono-0.4.23/src/offset/mod.rs:189:17

Waiting until #883 makes it in, the best bet I currently found as a workaround is to extract the implementation from there:

pub fn midnight_on<Tz>(date: chrono::NaiveDate, tz: &Tz) -> chrono::DateTime<Tz>
where
    Tz: Clone + std::fmt::Debug + chrono::TimeZone,
{
    let base = chrono::NaiveTime::MIN;
    for multiple in 0..=24 {
        let start_time = base + chrono::Duration::minutes(multiple * 15);
        match date.and_time(start_time).and_local_timezone(tz.clone()) {
            chrono::LocalResult::None => continue,
            chrono::LocalResult::Single(dt) => return dt,
            chrono::LocalResult::Ambiguous(dt1, dt2) => {
                if dt1.naive_utc() < dt2.naive_utc() {
                    return dt1;
                } else {
                    return dt2;
                }
            }
        }
    }

    panic!(
        "Unable to calculate start time for date {} and time zone {:?}",
        date, tz
    )
}

[pitdicker]

In my opinion the problem is that we have no way to resolve a local time that falls inside a timezone transition gap (with LocalResult::None as result) to something useful. Fixing this requires a breaking change to LocalResult.

#716 is the issue to track that.