[GitHub] [arrow] MarcoGorelli commented on issue #36110: [C++][Python] Wrong result when converting time zones after 2038

["MarcoGorelli (via GitHub)" <gi...@apache.org>]

MarcoGorelli commented on issue #36110:
URL: https://github.com/apache/arrow/issues/36110#issuecomment-1594774767

   thanks for looking into this so deeply!
   
   sure but the zoneinfo one still looks more correct? e.g. in the UK the DST transition happens on the last Sunday of October and of March, and (unfortunately) hasn't announced that they intend to change this. zoneinfo seems to extrapolate that correctly beyond 2038:
   
   ```python
   In [29]:
       ...: string = '2058-10-28 00:00:00.000000'
       ...:
       ...: dt = datetime.fromisoformat(string)
       ...: dt = dt.replace(tzinfo=ZoneInfo('Europe/London'))
       ...: tz = ZoneInfo('UTC')
       ...: converted_dt = dt.astimezone(tz)
       ...: print(converted_dt)
   2058-10-28 00:00:00+00:00
   
   In [30]:
       ...: string = '2058-10-27 00:00:00.000000'
       ...:
       ...: dt = datetime.fromisoformat(string)
       ...: dt = dt.replace(tzinfo=ZoneInfo('Europe/London'))
       ...: tz = ZoneInfo('UTC')
       ...: converted_dt = dt.astimezone(tz)
       ...: print(converted_dt)
   2058-10-26 23:00:00+00:00
   
   In [31]: dt.strftime('%a')
   Out[31]: 'Sun'
   ```
   
   For reference, same with chrono-tz (haven't looked into how, but this is just to say - the fact `chrono-tz` extrapolates forwards differently to `arrow` explains the discrepancy when converting polars to pyarrow (or to pandas))
   ```Rust
   use chrono::{NaiveDateTime, TimeZone};
   use chrono_tz::Tz;
   
   fn main() {
       let dt = NaiveDateTime::parse_from_str("2058-10-28 00:00:00", "%Y-%m-%d %H:%M:%S").unwrap();
       let tz = Tz::Europe__London;
       let dt = tz.from_local_datetime(&dt).unwrap();
       println!("converted: {:?}", dt);
       let dt = NaiveDateTime::parse_from_str("2058-10-27 00:00:00", "%Y-%m-%d %H:%M:%S").unwrap();
       let tz = Tz::Europe__London;
       let dt = tz.from_local_datetime(&dt).unwrap();
       println!("converted: {:?}", dt);
   }
   ```
   outputs
   ```
   converted: 2058-10-28T00:00:00GMT
   converted: 2058-10-27T00:00:00BST
   ```
   
   ---
   
   copied from https://github.com/apache/arrow/issues/15047#issuecomment-1594706777


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