panic with Local Timezone 2023/11/29 02:00:00 checked_sub_months about chrono

[pitdicker]

If your local timezone is in Europe (like me) then the result should be 2023-10-29 01:04:43. That is right in the transition from summer to winter time, which is documented to return None.

I don't think checked_sub_months has a very sane API. It falls back to another day when the target month is shorter than the source month. Yet it returns None on something less significant such as a DST transition 😞. Hopefully something we can fix with chrono 0.5.

If you want to get a LocalResult instead of Option you can try:

#[test]
fn test_unwrap_naive_local_now() {
    let datetime = Utc.with_ymd_and_hms(2023, 11, 29, 01, 04, 43).unwrap();
    // Local Timezone is UTC+1
    let local_now = datetime.with_timezone(&Local);
    let naive_now = local_now.naive_local();

    let naive_last_month = naive_now.checked_sub_months(Months::new(1)).unwrap();
    let local_last_month = Local.from_local_datetime(&naive_last_month);

    assert!(matches!(local_last_month, LocalResult::Ambiguous(_, _)));
}

[truehumandesign]

@pitdicker thank you very much for the explanation and providing a solutions for that problem