The Week That Started A Day Late Debugging Lab

The Week That Started A Day LateDate/TimeMediumPreparing workspaceAutosaved

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Incident Dossier

The Week That Started A Day Late

A weekly summary service buckets events by Monday-Sunday weeks. Every event from Monday through Friday shows up in the *previous* week's bucket, and Sunday events appear in the *current* week's bucket. Engineers confirm the server timezone is UTC and that the function is given an injected `now` — there is no real-clock involvement. Something in the arithmetic is just one day off.

Initial failing signalstartOfWeek(Thursday) returns the prior Sunday instead of MondayExpected 2026-07-27T00:00:00Z (Monday) · received 2026-07-26T00:00:00Z (Sunday)

Progress

1Reproduce the bugRun checks

2Edit the fixOpen

3Pass checksOpen

4Submit fixOpen

Reproduction Steps

Run npm test.
Skim src/week.js and identify how the function walks backward from the given timestamp.
Trace which day of week `getUTCDay()` returns for a Thursday and what gets subtracted.
Apply the smallest fix that keeps Sunday rolling back to the PRECEDING Monday, not the same Sunday.

Focus Mode

Start

src/week.js

Likely area

src/week.js

Progressive hints

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

TopicDate/Time

DifficultyMedium

Est. time18 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...