fix(ui): bucket calendar issues by their real day in non-UTC timezones

[martian56]

Summary

Closes #140.

In the work-item Calendar layout, issues were bucketed into the wrong day cell for anyone outside UTC. Two date round-trips were to blame:

• Each cell's lookup key was built as isoDate(date.toISOString().slice(0, 10)). The cell date is a local-midnight Date, so for a positive-offset client toISOString() rolls back to the previous UTC day and the cell key ends up one day early. Every issue then rendered in the next cell (a due-Jun-20 issue landed in the Jun-21 cell), and the per-day counts and "+N more" were off too.
• isoDate parsed the target_date ("YYYY-MM-DDT00:00:00Z") into a Date and read its local components, which shifts negative-offset clients the other way.

Fix:

• Cell keys now come from the date's local components (localKey), i.e. the calendar day the cell actually represents.
• isoDate now takes the leading YYYY-MM-DD of target_date directly (the calendar day the backend stored), with no Date round-trip.

Buckets and cells agree on the calendar day in every timezone now.

Testing

• npm run typecheck and npm run lint pass.
• Browser-verified in Asia/Baku (UTC+4) — the reported repro timezone. In June 2026, "DUE-20th" (target_date 2026-06-20) now renders in the day-20 cell and "Child of Alpha" (2026-06-26) in the day-26 cell. Before the fix both were shifted +1 (day-21 / day-27).

AI assistance

This change was produced with the help of Claude Code (Claude Opus 4.8). See the Co-Authored-By trailer on the commit.

Summary by CodeRabbit

• Bug Fixes
  • Fixed calendar day cells to use the correct local date key, reducing mismatches in day-specific issue views.
  • Improved date handling for calendar entries so UTC-based timestamps display consistently across time zones.

[strix-security]

Strix Security Review

No security issues found.

_{Updated for 9ab76db.}

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 29334e5f-42a6-416f-8d69-e48a4bb51c63

📥 Commits

Reviewing files that changed from the base of the PR and between c370080 and 9ab76db.

📒 Files selected for processing (1)

• apps/web/src/components/work-item/layouts/IssueLayoutCalendar.tsx

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📝 Walkthrough

Walkthrough

The calendar layout's per-day cell key generation was changed from an ISO-string round-trip to a local date-key helper, and the isoDate function was rewritten to extract YYYY-MM-DD via regex instead of Date.parse, addressing a timezone-related off-by-one day bug.

Changes

Calendar day-key fix

Layer / File(s)	Summary
Local date-key helper and cell keying apps/web/src/components/work-item/layouts/IssueLayoutCalendar.tsx	Day-cell keys now use a new localKey(date) helper instead of isoDate(date.toISOString()...); isoDate is reworked to extract YYYY-MM-DD via regex with documentation on UTC-midnight timestamp assumptions.

Estimated code review effort: 2 (Simple) | ~10 minutes

Poem

A hop, a skip, one day undone,
No more shifting in the sun,
Local keys now light the way,
Each issue lands on its rightful day,
🐇📅 Calendar cells, aligned and true!

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title is specific, concise, and matches the timezone day-bucketing fix.
Description check	✅ Passed	The description covers the bug, fix, linked issue, testing, and AI disclosure, though it omits several optional template sections.
Linked Issues check	✅ Passed	The change matches #140 by using local day keys and avoiding date round-trips that shifted issues by one day in non-UTC timezones.
Out of Scope Changes check	✅ Passed	The diff stays confined to the Calendar layout date-keying fix and related helper logic, with no unrelated changes.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/calendar-timezone-off-by-one

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