feat(loops): Customize timezone for loop schedules

[MattPua]

Problem

Why: Loop schedules default to UTC and provide little context about when they will run, making configured times surprising outside UTC.

Users want to be able to set loops scheduled in their timezone

Changes

Before:

After:

• Default new loop schedules and templates to the detected system timezone
• Add a timezone picker consistent with the rest of the trigger form
• Show the next run while creating a schedule and in loop details
• Add reusable timezone timestamp, conversion popover, and recurrence utilities
• Add copy feedback and Storybook coverage for timezone displays

How did you test this?

• pnpm --filter @posthog/ui typecheck
• pnpm --filter @posthog/ui exec vitest run src/features/loops/loopDisplay.test.ts src/features/loops/loopCron.test.ts src/features/loops/loopFormTypes.test.ts src/primitives/timezone.test.ts src/primitives/TimezoneConversionTooltip.test.tsx
• Storybook production build
• git diff --check

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

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Created with PostHog Code

[trunk-io]

😎 This pull request was merged.

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 9436f41.}

[posthog]

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[greptile-apps]

Comments Outside Diff (1)

1. packages/ui/src/features/loops/components/LoopTriggerEditor.tsx, line 302 (link)

   Frequency Toggle Loses Timezone

   Switching a recurring trigger to Once replaces its config with only run_at, discarding the selected timezone. If the user switches back to recurring, the editor falls back to systemTimezone() and saves that value, so a schedule configured for Europe/London can silently become America/Toronto and run at a different instant.

   Prompt To Fix With AI

   This is a comment left during a code review.
   Path: packages/ui/src/features/loops/components/LoopTriggerEditor.tsx
   Line: 302
   
   Comment:
   **Frequency Toggle Loses Timezone**
   
   Switching a recurring trigger to Once replaces its config with only `run_at`, discarding the selected timezone. If the user switches back to recurring, the editor falls back to `systemTimezone()` and saves that value, so a schedule configured for `Europe/London` can silently become `America/Toronto` and run at a different instant.
   
   How can I resolve this? If you propose a fix, please make it concise.

Prompt To Fix All With AI

Fix the following 3 code review issues. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 3
packages/ui/src/features/loops/components/LoopTriggerEditor.tsx:302
**Frequency Toggle Loses Timezone**

Switching a recurring trigger to Once replaces its config with only `run_at`, discarding the selected timezone. If the user switches back to recurring, the editor falls back to `systemTimezone()` and saves that value, so a schedule configured for `Europe/London` can silently become `America/Toronto` and run at a different instant.

### Issue 2 of 3
packages/ui/src/features/loops/components/LoopTriggerEditor.tsx:264
**Missing Timezone Changes Meaning**

An existing recurring trigger with no stored timezone is interpreted as UTC by `nextScheduleRun`, but the editor now presents it as the device timezone. Editing any recurrence field then persists that device timezone, silently changing when the existing loop runs; missing timezones should remain UTC while only new triggers default to `systemTimezone()`.

### Issue 3 of 3
packages/ui/src/primitives/timezone.ts:80-83
**Fallback Removes Most Timezones**

When `Intl.supportedValuesOf` is unavailable, this fallback leaves the picker with only the device timezone and UTC. In a supported host runtime without that optional API, users cannot select any other valid IANA timezone, so the new scheduling feature is unusable for schedules targeting another region.

_{Reviews (1): Last reviewed commit: "fix: prevent timezone popover reopening" | Re-trigger Greptile}