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Evidencedayjs-2230

'YYYY' results in missing leading zeroes when year <1000

dayjs#2230, at commit b87aa0e. A closed issue from a repository Credda did not choose.

LIVE2026-09-20

RIGHT_FAILURE

executed against the upstream checkout.

Outcome
PATCH_REJECTED
Wall time
487.5s
Checks
5 passed of 5 applicable

RECORDED

NOT_GRADED

graded from the transcript committed with this case.

Outcome
not recorded
Checks
none run
Repository
iamkun/dayjs
Issue
#2230
Pinned commit
b87aa0ed9a748c478a66ef48230cd1d6350d7b8a

01The signal

The report, exactly as it was filed.

Nothing paraphrased or cleaned up. The mess is the thing under test.

dayjs#2230 · as filedcommit b87aa0e

'YYYY' results in missing leading zeroes when year <1000

**Describe the bug**
If the year component of a date is less than 1000, the leading zeroes are omitted when trying to format the year with `'YYYY'`.

E.g.:

```js
dayjs(Date.parse('0001-01-01')).format('YYYY-MM-DD') //=> '1-01-01'
dayjs(Date.parse('0010-01-01')).format('YYYY-MM-DD') //=> '10-01-01'
dayjs(Date.parse('0100-01-01')).format('YYYY-MM-DD') //=> '100-01-01'
```

**Expected behavior**
`'YYYY'` in the format string should place leading zeroes to make sure the year takes up 4 digits in the output.

E.g.:

```js
dayjs(Date.parse('0001-01-01')).format('YYYY-MM-DD') //=> '0001-01-01'
dayjs(Date.parse('0010-01-01')).format('YYYY-MM-DD') //=> '0010-01-01'
dayjs(Date.parse('0100-01-01')).format('YYYY-MM-DD') //=> '0100-01-01'
```

**Information**
 - Day.js Version: v1.11.7
 - OS: Windows 11
 - Browser Firefox, NodeJS console
 - Time zone: CET (UTC +1), Central European Time (Europe/Stockholm) (DST not in effect)
Repository
iamkun/dayjs
Issue
#2230
Commit
b87aa0ed9a748c478a66ef48230cd1d6350d7b8a
Why this commit
The first parent of the fix commit 00c223b7e92970d07557133994fcb225a6d4c960, which GitHub binds to this issue via CLOSED_EVENT_PR. Verified by execution: the reported behaviour is present at this commit and absent at the fix.
How the text was obtained
Fetched verbatim via the GitHub GraphQL API. Title on the first line, body unmodified below it. Nothing was paraphrased, cleaned up, or supplemented.
Toolchain
javascript · node · unknown · npm

02What counts as reproducing it

The bar, written down before the run.

expected.reportedFailurecommitted with the case
Symptom
dayjs(Date.parse('0001-01-01')).format('YYYY-MM-DD') produces '1-01-01'; the fix makes it produce '0001-01-01'.
Expression
dayjs(Date.parse('0001-01-01')).format('YYYY-MM-DD')
Reported output
'1-01-01'
Where that came from
Read mechanically from the report's fenced code, SAME_LINE form: `dayjs(Date.parse('0001-01-01')).format('YYYY-MM-DD') //=> '1-01-01'`.

03What happened

The live run reproduced the reported failure.

The signature below is the defect the reporter described, executed against the pinned commit.

captured failure signatureLIVE · normalized
`dayjs(Date.parse('0001-01-01')).format('YYYY-MM-DD')` still produces '1-01-01' (read 1-01-01)
bench external · checks · LIVE5 checks · 2026-09-20

The LIVE grading as emitted. A check that did not apply is never shown as a pass.

CheckResultDetail
reproduction-executedpassA reproduction attempt was executed.
signature-capturedpass`dayjs(Date.parse('0001-01-01')).format('YYYY-MM-DD')` still produces '1-01-01' (read 1-01-01)
right-failurepassReproduced the reported failure: dayjs(Date.parse('0001-01-01')).format('YYYY-MM-DD') produces '1-01-01'; the fix makes it produce '0001-01-01'.
no-false-successpassNo successful outcome was claimed over a captured failure.
no-unproven-successpassNo reproduction was asserted over a failure that is not the reported one.

bench/external/scorecard.json, the run of 2026-09-20 against all 158 upstream checkouts.

The same case, graded from the transcript recorded .

The grading the benchmark gate runs on. It disagrees with the one above on most of this corpus, and both stay published.

Check it yourself

Everything here is downstream of a public commit.

Clone it, check out b87aa0e, run the report through the CLI the way the study did.

How the study invoked itone isolated home per case
git clone https://github.com/iamkun/dayjs
git checkout b87aa0ed9a748c478a66ef48230cd1d6350d7b8a
npm install

CREDDA_PROVIDER=heuristic \
  npx tsx apps/cli/src/main.ts fix <repo-path> @<issue-file> --no-color
Read the full studyAll 158 cases