Summary

Fix incorrect ordering comparisons for DateTime instances during DST fold transitions.

During the repeated hour after a daylight saving time (DST) rollback, two datetimes can represent different UTC instants while sharing the same timezone object. In this situation, ordering comparisons (<, <=, >, >=) can return results that do not reflect the actual chronological order.

This change normalizes both operands to UTC before performing ordering comparisons, ensuring that comparisons are based on the represented instant rather than the ambiguous local wall-clock time.

Problem

The issue can be reproduced with datetimes created during the DST fall-back transition:

d1 = pendulum.datetime(
    2023, 11, 5, 1, 15,
    tz="America/Los_Angeles",
    fold=1,
)

d2 = pendulum.datetime(
    2023, 11, 5, 1, 25,
    tz="America/Los_Angeles",
    fold=0,
)

print(d1.timestamp() > d2.timestamp())  # True
print(d1 < d2)                          # True (incorrect)

Although d1 represents a later UTC instant than d2, the ordering comparison reports the opposite result.

After reproducing the issue locally, I verified that the problem only affects relational ordering operators. Equality behavior remains unchanged.

Solution

The ordering operators now normalize both operands to UTC before performing the comparison.

The implementation:

• normalizes both operands to UTC using astimezone(),
• preserves existing equality semantics,
• continues to support comparisons with standard datetime.datetime instances,
• returns NotImplemented for unsupported comparison types,
• ensures ordering comparisons reflect the actual UTC instant represented by each object.

The change is intentionally limited to ordering comparisons and does not modify equality behavior.

Why this approach?

Pendulum's comparison documentation states that comparisons are performed in UTC. Normalizing both operands before ordering comparisons keeps the implementation aligned with that documented behavior while limiting the scope of the change to relational operators only.

Testing

Added a regression test covering the DST fold scenario:

• test_less_than_with_fold

Validation performed locally:

• tests/datetime/test_comparison.py — 37 passed
• tests/datetime — 565 passed

This change is intentionally scoped to ordering comparisons and does not modify equality semantics.

Fixes #855