[GitHub] [arrow-rs] tustvold commented on pull request #4201: Fix incorrect cast Timestamp with Timezone

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

tustvold commented on PR #4201:
URL: https://github.com/apache/arrow-rs/pull/4201#issuecomment-1543889159

   Thank you for this, I'm not sure this logic is correct, however.
   
   If both quantities have timezones, no adjustment should be necessary, as they are already normalized to UTC.
   
   The complexity arises when converting between timestamps where one doesn't have a timezone. This requires some care, especially to handle timezones that don't have fixed offsets.
   
   In the forward direction I think this involves doing something like
   
   ```
   let from_tz = from_tz.parse()?;
   array.unary_opt(|i| {
       T::make_value(as_datetime_with_timezone(i, from_tz)?.naive_local())
   })
   ```
   
   And in the reverse direction
   
   ```
   let to_tz = to_tz.parse()?;
   array.unary_opt(|i| {
       let local = as_datetime(i);
       let offset = to_tz.offset_from_local_datetime(&local).single()?;
       T::make_value(local + offset.fix())
   })
   ```


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