fix(datetime): subtract 1 from ACARS month before setUTCMonth (#424)

[SAY-5]

Summary

Fixes #424. UTCDateTimeToString passed the raw ACARS month digit (1-12) to Date.prototype.setUTCMonth, which expects 0-11. Every decoded date came out one month forward, 150226 (15 Feb 2026) decoded to 15 March 2026.

While addressing the off-by-one I also folded the construction into a single new Date(Date.UTC(...)) call. The original incremental setUTC* sequence is fragile when the system clock falls on a day that doesn't exist in the target month: setting day=28 on a Date already at the 31st silently rolls forward, so encoding 280226 while the host clock said 31 March would have decoded to 28 March.

Test plan

• npx jest, full suite: 410 passed, 9 skipped, 89 suites
• New DateTimeUtils.test.ts:
  • decodes a DDMMYY date with the ACARS 1-12 month convention, direct regression for Bug: DateTimeUtils.UTCDateTimeToString month is off-by-one #424
  • does not roll the month forward when the system date is later than the target month-end, uses jest.setSystemTime to pin a 31-day-month system clock and verifies the order-fragility fix
  • handles a six-digit time with seconds, exercises the HHMMSS branch

Summary by CodeRabbit

• Bug Fixes

  • Fixed UTC date/time parsing and formatting, correcting month indexing, ensuring proper year fallback, and stabilizing seconds handling.

• Tests

  • Added deterministic tests for date/time utilities, covering month-end rollover and time-string formats (including seconds).

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: ec99d2ea-58bb-4fec-abe9-89560c5bc7da

📥 Commits

Reviewing files that changed from the base of the PR and between 1dc9384 and 11211f7.

📒 Files selected for processing (1)

• lib/DateTimeUtils.test.ts

✅ Files skipped from review due to trivial changes (1)

• lib/DateTimeUtils.test.ts

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Walkthrough

Fixes an off-by-one month bug in UTCDateTimeToString by building the UTC Date via Date.UTC(...) with month adjusted from ACARS 1–12 to JS 0–11; adds deterministic Jest tests including a month-end rollover regression and seconds-aware time parsing.

Changes

UTC date parsing and tests

Layer / File(s)	Summary
Data Shape / Parsing lib/DateTimeUtils.ts	Replaces incremental setUTC* mutations with a single Date.UTC(year, month-1, day, hour, minute, second) construction; subtracts 1 from parsed month; when dateString.length !== 6 uses current UTC year; only parses seconds when timeString.length === 6.
Entry / API lib/DateTimeUtils.ts (same file, different ranges)	UTCDateTimeToString(dateString, timeString) now derives all UTC components from inputs and returns the formatted UTC string from the constructed Date.
Deterministic Regression Test lib/DateTimeUtils.test.ts	New Jest suite verifying: correct DDMMYY decoding (month/day/year), month-end rollover fixed by pinning system time to 2026-03-31 ensuring 280226 yields Feb 28 2026, and time parsing accepts HHMMSS (verifies 12:30:45).

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Suggested reviewers

• kevinelliott

Poem

🐰 I nibbled bugs by moonlit light,
Subtracted one to make months right,
One Date.UTC call, tidy and neat,
Tests pinned steady, no more sleight—
February hops back in place tonight.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main fix: subtracting 1 from ACARS month before setUTCMonth to correct the off-by-one bug.
Linked Issues check	✅ Passed	The PR fully addresses issue #424 by correcting month handling (subtract 1) and rebuilding with Date.UTC() for safer date construction, resolving the off-by-one bug described.
Out of Scope Changes check	✅ Passed	All changes directly address the linked issue: month correction in DateTimeUtils.ts, robust date construction via Date.UTC, and comprehensive regression tests in DateTimeUtils.test.ts.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[makrsmark]

I don't understand why we're setting system time. Can you explain?