# Implementation Plan - Fix RRULE UNTIL Conversion

## Phase 1: Reproduction [checkpoint: 02fcb7e]
- [x] Task: Reproduce `Invalid UNTIL value` error 17de604
    - [x] Add a test case in `icalendar_test.ts` where `rrule` object has an `until` property as a `Date`.
    - [x] Run the test and confirm it fails with `Error: Invalid UNTIL value: [object Object]`.
- [x] Task: Conductor - User Manual Verification 'Reproduction' (Protocol in workflow.md)

## Phase 2: Fix Implementation
- [ ] Task: Implement value formatting logic in `icalendar.ts`
    - [ ] Update `expandRecurrences` to use a helper for property value conversion.
    - [ ] Ensure `Date` objects are formatted as `YYYYMMDDTHHMMSSZ`.
    - [ ] Run the test to confirm it passes.
- [ ] Task: Conductor - User Manual Verification 'Fix Implementation' (Protocol in workflow.md)

## Phase 3: Verification & Cleanup
- [ ] Task: Full Regression Check
    - [ ] Run all tests in `icalendar_test.ts`.
- [ ] Task: Conductor - User Manual Verification 'Verification & Cleanup' (Protocol in workflow.md)