The Invoice That Stuck Around For An Extra Month Debugging Lab

The Invoice That Stuck Around For An Extra MonthDate/TimeHardPreparing workspaceAutosaved

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Incident Dossier

The Invoice That Stuck Around For An Extra Month

A billing service computes the next invoice date by adding N months to the customer's billing anchor and clamping to the last day of the target month when the day is shorter. Mar 31 + 1 month should become Apr 30, Dec 15 + 2 months should become Feb 15 of the following year, and Jan 31 + 1 month should collapse to Feb 28 (or Feb 29 in leap years). The clamp helper is right; the result formatter is reading a month index that's already been corrected once — and so every invoice is dated one month LATER than what the math intended.

Initial failing signaladdMonths(2026, 3, 31, 1) returns 2026-05-30 instead of 2026-04-30Expected 2026-04-30 (Apr 30) · received 2026-05-30 (May 30)

Progress

1Reproduce the bugRun checks

2Edit the fixOpen

3Pass checksOpen

4Submit fixOpen

Reproduction Steps

Run npm test.
Read src/billing-cycle.js line by line, paying attention to which helper receives the zero-indexed month and which receives the one-indexed month.
Verify with the visible tests (Mar 31 + 1 month, Dec 15 + 2 months) and then reason about Jan 31 + 1 month.
Fix the one place where a 0-indexed value is passed somewhere expecting a 1-indexed value (or vice versa).

Focus Mode

Start

src/billing-cycle.js

Likely area

src/billing-cycle.js

Progressive hints

Hints reveal broad direction first, then narrow toward the suspect path without showing hidden tests or exact solution code.

TopicDate/Time

DifficultyHard

Est. time26 min

Workspacecreating

Explorer

Preparing workspace files…

Evidence Map

Failing signal loaded from challenge API
Workspace files loaded from workspace API
Results rendered only after API run/submit

Autosaved

workspace

Outputread-onlyready$ workspace boot

Workspace booting...