fix: apply recurring DST correction in the event timezone by winklemad · Pull Request #29777 · calcom/cal.diy

winklemad · 2026-07-14T18:42:02Z

What does this PR do?

parseRecurringDates (packages/lib/parse-dates.ts) computes its per-occurrence DST correction from the runtime's local timezone rather than the event timeZone:

const startUtcOffset = dayjs(startDate).utcOffset();          // runtime-local offset
// ...
return dayjs.utc(t).add(startUtcOffset - dayjs(t).utcOffset(), "minute");  // runtime-local offset

The booker lets a user pick a meeting timezone independent of their browser timezone, so browser TZ ≠ meeting TZ is normal input. When they differ, every recurring occurrence after the event zone's DST transition drifts by an hour, and these instants are used as the actual booking start/end — parseRecurringDates's returned Date[] feeds mapRecurringBookingToMutationInput (packages/features/bookings/lib/client/booking-event-form/booking-to-mutation-input-mapper.tsx:107) and the occurrence list in Occurences.tsx. So this is wrong booking times, not just a display glitch.

Reproduced with a weekly 10:00 America/New_York booking crossing US spring-forward (2025-03-09), booker runtime Asia/Dubai (each instant shown as its New-York wall clock):

before: 10:00, 11:00, 11:00, 11:00 ❌ (post-DST occurrences drift +1h)
after: 10:00, 10:00, 10:00, 10:00 ✅

When the runtime timezone equals the event timezone the current code is already correct — confirming the intent is to hold the event-timezone wall-clock, so the cross-timezone drift is a genuine bug.

Fix: read both offsets in the event timeZone (.tz(timeZone).utcOffset()), matching the function's own comment ("UTC still need to have DST applied").

No linked issue — found by reading the code.

Visual Demo (For contributors especially)

Non-visual (timezone/scheduling logic). The reproduction is the code + before/after above.

Mandatory Tasks (DO NOT REMOVE)

I have self-reviewed the code (A decent size PR without self-review might be rejected).
N/A — this fix does not require a developer-docs change.
I confirm automated tests are in place that prove my fix is effective (packages/lib/parse-dates.timezone.test.ts).

How should this be tested?

packages/lib/parse-dates.timezone.test.ts (runs in the VITEST_MODE=timezone suite; sets process.env.TZ = "Asia/Dubai") asserts a weekly 10:00 America/New_York recurrence crossing the 2025-03-09 DST boundary keeps all four occurrences at 10:00. It fails on the current code (occurrences 2–4 become 11:00) and passes with the fix.

No environment variables needed.
No test data / DB needed — parseRecurringDates is a pure function.
Expected: ["2025-03-05 10:00","2025-03-12 10:00","2025-03-19 10:00","2025-03-26 10:00"] (New-York wall clock), regardless of the runtime timezone.

parseRecurringDates computed its per-occurrence DST correction from the runtime's local timezone (dayjs(startDate).utcOffset() and dayjs(t).utcOffset()) instead of the event timeZone. When a booker's browser timezone differs from the meeting timezone, every recurring occurrence after the event zone's DST transition drifted by an hour (e.g. a 10:00 America/New_York weekly slot became 11:00), and those instants are used as the actual booking start/end. Read both offsets in the event timeZone so the wall-clock time stays constant across the DST change.

github-actions · 2026-07-14T18:42:10Z

Welcome to Cal.diy, @winklemad! Thanks for opening this pull request.

A few things to keep in mind:

This is Cal.diy, not Cal.com. Cal.diy is a community-driven, fully open-source fork of Cal.com licensed under MIT. Your changes here will be part of Cal.diy — they will not be deployed to the Cal.com production app.
Please review our Contributing Guidelines if you haven't already.
Make sure your PR title follows the Conventional Commits format.

A maintainer will review your PR soon. Thanks for contributing!

coderabbitai · 2026-07-14T18:44:53Z

No actionable comments were generated in the recent review. 🎉

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packages/lib/parse-dates.ts

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packages/lib/parse-dates.ts

📝 Walkthrough

Walkthrough

parseRecurringDates now calculates initial and per-occurrence UTC offsets using the event’s configured timezone. A Vitest regression test sets the runtime timezone to Asia/Dubai and verifies that a weekly recurrence in America/New_York remains at 10:00 across the 2025 spring-forward DST transition.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly matches the main change: applying DST correction in the event timezone.
Description check	✅ Passed	The description directly explains the timezone/DST bug, the fix, and the added test, all matching the changeset.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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Actionable comments posted: 1

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Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@packages/lib/parse-dates.ts`:
- Around line 111-112: Remove the redundant “applying the DST offset” comment
immediately before the return statement in the date-parsing logic, leaving the
existing calculation unchanged.

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packages/lib/parse-dates.timezone.test.ts
packages/lib/parse-dates.ts

github-actions · 2026-07-22T00:38:28Z

This PR has been marked as stale due to inactivity. If you're still working on it or need any help, please let us know or update the PR to keep it active.

CLAassistant · 2026-07-23T03:45:00Z

All committers have signed the CLA.

pull-request-size Bot added the size/M label Jul 14, 2026

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Comment thread packages/lib/parse-dates.ts Outdated

Remove redundant what-comment in parseRecurringDates

6e40c02

github-actions Bot added the Stale label Jul 22, 2026

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fix: apply recurring DST correction in the event timezone#29777

fix: apply recurring DST correction in the event timezone#29777
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winklemad commented Jul 14, 2026

What does this PR do?

Visual Demo (For contributors especially)

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