Make history time-window tests portable across non-hour UTC offsets

[codeforester]

Goal

Make the Base history test suite pass in every supported host timezone, including zones with half-hour or quarter-hour offsets.

Background

parse_history_bound correctly treats naive short timestamps as host-local time and converts them to UTC. The regression test only asserts that the converted minute remains 15, which is false in Asia/Kolkata: 2026-06-10 10:15 local correctly becomes 04:45 UTC.

The focused test passes under TZ=UTC and fails under TZ=Asia/Kolkata. This causes the supported ./bin/base-test path to fail locally while hosted UTC CI remains green.

Relevant code:

• https://github.com/basefoundry/base/blob/main/cli/python/base_history/engine.py#L252-L270
• https://github.com/basefoundry/base/blob/main/cli/python/base_history/tests/test_engine.py#L236-L240

Related issue:

• Add chronological ordering and time-window filters to basectl history #1676

Scope

• Replace the UTC-biased assertion with a deterministic host-timezone expectation.
• Add explicit coverage for a non-whole-hour UTC offset.
• Preserve the public rule that naive short forms use host-local time and explicit offsets take precedence.
• Keep canonical JSON timestamps in UTC.

Acceptance Criteria

• The focused test passes under UTC, Asia/Kolkata, and at least one negative UTC offset.
• Tests assert the complete expected instant rather than one coincidentally stable field.
• Explicit Z and offset-bearing inputs remain unchanged.
• The full supported test harness passes in the host timezone.

Validation

• Run the focused test under TZ=UTC.
• Run the focused test under TZ=Asia/Kolkata.
• Run the complete base_history test module.
• env -u BASE_HOME ./bin/base-test
• git diff --check

Non-Goals

• Do not change the documented parsing semantics.
• Do not force all users to enter UTC.
• Do not change timestamp rendering options.

Project Fields

• Status: Ready
• Priority: P2
• Area: Python
• Initiative: Contract Hardening
• Size: S

Agent Assignment

• Assignee: codeforester