Case study: dayjs localizedFormat
No upstream issue was found for this one. It is a found-in-the-wild failure: on iamkun/dayjs at 436bde0 (dev, 2026-09-15), test/plugin/localizedFormat.test.js fails in any time zone west of UTC and passes elsewhere. test/plugin/dayOfYear.test.js fails the same way (same cause: fixtures written as UTC instants read back as local dates); only the first is covered here.
The test formats new Date(0) and expects the year 1970. In America/New_York that instant is 1969-12-31 19:00 local, so dayjs correctly prints 1969.
| File | What it is |
|---|---|
new-york.json |
capsule from a reporter in America/New_York: the test fails |
kolkata.json |
capsule from a maintainer in Asia/Kolkata: the test passes |
replay.txt |
twin replay new-york.json on Solari, recorded 2026-09-29 |
bisect.txt |
twin bisect new-york.json --good kolkata.json on Solari, recorded 2026-09-29 |
fix.patch |
candidate fix for the test |
verify.txt |
twin verify new-york.json --patch fix.patch on Solari, recorded 2026-09-29 |
Both capsules: Node 26.7.0, npm 12.0.2 (npm ci from package-lock.json), captured with LANG=C.UTF-8, no LC_* and NO_COLOR=1.
What happened
Section titled “What happened”Capture, from the repository root:
npx jest --coverage=false test/plugin/localizedFormat.test.jsTZ=America/New_York fails (Expected "1970 l 1970", Received "1969 l 1969"), TZ=Asia/Kolkata passes. twin diff shows only the time zone.
| Command | Verdict | Wall time |
|---|---|---|
twin replay new-york.json |
REPRODUCED (3 of 3 attempts, same signature) | 46 s |
twin bisect new-york.json --good kolkata.json |
minimal difference TZ=America/New_York |
44 s |
twin verify new-york.json --patch fix.patch |
FIXED (3 of 3 attempts pass) | 45 s |
fix.patch builds the date with new Date(1970, 0, 1, 12) (local noon) so the year is 1970 in every zone. It passes locally in UTC, New York, Los Angeles, Kolkata, Auckland and Kiritimati. Wall times are the whole command; npm ci is about 25 s of that.
Run it yourself
Section titled “Run it yourself”export SOLARI_API_KEY=...twin replay examples/dayjs-localizedformat-tz/new-york.jsontwin bisect examples/dayjs-localizedformat-tz/new-york.json --good examples/dayjs-localizedformat-tz/kolkata.jsontwin verify examples/dayjs-localizedformat-tz/new-york.json --patch examples/dayjs-localizedformat-tz/fix.patchEvery file referenced here is in examples/dayjs-localizedformat-tz.