spicysharkco opened a new pull request, #11702:
URL: https://github.com/apache/arrow-rs/pull/11702
# Which issue does this PR close?
- Closes #11698
# Rationale for this change
An RFC 3339 offset has no seconds, so chrono rounds one that is not a whole
number of minutes, and the text names a different instant:
```rust
let array =
TimestampMillisecondArray::from(vec![-302443199750]).with_timezone("Africa/Monrovia");
// formatted with the default `FormatOptions`:
// 1960-06-01T11:15:30.250-00:45
```
The stored instant is `1960-06-01T12:00:00.250Z`, so the text is 30 seconds
later. Africa/Monrovia was 44 minutes 30 seconds behind UTC until 1972, and
most zones' local mean time is affected too. The CSV and JSON writers and casts
to a string type format through
[`write_timestamp`](https://github.com/apache/arrow-rs/blob/3327fbef1c3381a149e9a413a5bb22813e11639e/arrow-cast/src/display.rs#L770),
so a reader gets a different instant with no error.
# What changes are included in this PR?
With the default format, when the offset at that instant is not a whole
number of minutes, `write_timestamp` writes the instant in UTC (`...Z`). That
is exact, and `string_to_datetime` reads it back; it rejects an offset with
seconds. Whole-minute offsets and custom formats are unchanged. The `Debug`
output of a timestamp array rounds the same way and is left as is.
# Are these changes tested?
Yes, `display::tests::test_timestamp_tz_offset_with_seconds`. It formats
Africa/Monrovia (1960) and Europe/Paris (1900), reads the text back to the same
instant, and checks that `+05:30` is still written as an offset. It fails on
`main`.
# Are there any significant user-facing changes?
The text for such an instant now ends in `Z` instead of a rounded offset.
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