Pull Request Check List

• Added tests for changed code.
• Updated documentation for changed code.

Summary

DateTime.diff() raises a TypeError when a naive DateTime is diffed against a naive stdlib datetime:

from datetime import datetime
import pendulum

pendulum.today().naive().diff(datetime.today())
# TypeError: can't compare offset-naive and offset-aware datetimes

Root cause

diff() builds an Interval(self, dt). Interval.__new__ validates that both endpoints have the same awareness (both naive or both aware) and otherwise raises. Interval.__init__, however, re-normalises the endpoints through pendulum.instance() using the default tz=UTC. For a naive stdlib datetime that turns it into a UTC-aware DateTime, so a naive receiver and the (now aware) argument end up mixed, and the subsequent start > end / precise_diff comparison raises.

Fix

Pass each endpoint's own tzinfo to instance() instead of letting it default to UTC:

pendulum.instance(start, tz=start.tzinfo)
pendulum.instance(end, tz=end.tzinfo)

DateTime.instance resolves the timezone as tz = dt.tzinfo or tz, so:

• naive inputs (tzinfo is None) stay naive — fixing the bug, and
• aware inputs keep their existing timezone — identical to the previous behaviour.

Because __new__ already guarantees both endpoints share the same awareness, forcing a timezone in __init__ was unnecessary and was the sole source of the mismatch. The mixed naive/aware case still raises in __new__ exactly as before.

Testing

• Added test_diff_naive_with_naive_stdlib_datetime covering the reported case (both abs=True and abs=False).
• Full test suite passes locally (1834 passed, 3 skipped).
• ruff check and ruff format --check pass on the changed files.

Fixes #880