[mmporong]

Summary

item() and dumps() render a datetime with isoformat(). When the UTC offset is not a whole number of minutes, isoformat() adds a seconds field (-00:44:30), and for an offset such as +00:00:30 the "+00:00" to "Z" replacement in item() turns it into Z:30. TOML offsets are HH:MM, so neither tomlkit nor tomllib can read the output back. zoneinfo gives these offsets (local mean time) for dates before a zone's first transition:

>>> from datetime import datetime
>>> from zoneinfo import ZoneInfo
>>> import tomlkit, tomllib
>>> s = tomlkit.dumps({"d": datetime(1960, 1, 1, 12, tzinfo=ZoneInfo("Africa/Monrovia"))})
>>> s
'd = 1960-01-01T12:00:00-00:44:30\n'
>>> tomllib.loads(s)
tomllib.TOMLDecodeError: Expected newline or end of document after a statement (at line 1, column 30)

This PR adds _format_datetime(), which writes such a value as the same instant in UTC (1960-01-01T12:44:30Z above) and leaves whole-minute offsets unchanged. item(), DateTime() without raw, and DateTime._new() (used by replace(), astimezone() and arithmetic) all go through it. The item keeps its original fields and tzinfo; only the written text changes, and the parsed value compares equal to the original. This is in the same spirit as #613, which made tz-aware time values render as valid TOML. If you would rather raise ValueError for these offsets, I can switch to that.

Tests:

• New tests in tests/test_items.py cover offsets of +30 s, +1 h 0 min 1 s, -44 min 30 s and 1 microsecond through item(), dumps(), astimezone() and replace(), and check that the item keeps its fields. All 5 fail on master and pass with this change.
• Full suite with the toml-test submodule: 1229 passed (1224 before, plus the 5 new tests), Python 3.11 only.
• The Africa/Monrovia example above, read back with tomllib, now gives a value equal to the original.
• ruff 0.17.2 (check and format --check): clean. mypy 1.19.1 (the pinned version): the same 33 errors before and after, none in the changed lines.

No linked issue.

Duplicate Check

• I searched the open PRs and found none that already fixes this issue.
• Related open PRs (if any, and why this PR is still needed): Make DateTime.astimezone timezone optional #651 and Preserve temporal serialization in reflected addition #652 change DateTime.astimezone() and reflected addition, not how the raw string is built. Reject UTC offsets TOML cannot write instead of emitting invalid output #598 (closed on 2026-09-08 without review; the author account no longer exists) proposed raising ValueError for these offsets; this PR writes them as UTC instead.

Agent Drafting Metadata

• Agent: Claude Code
• Model: Claude Opus 5.5
• Notes: The defect was found by an automated round-trip check (dumps() then tomllib.loads()) in an AI-assisted scan of open-source projects. The agent wrote the fix, the tests, the changelog entry and this description, and ran the checks listed above.