Bulk import reads a naive datetime cell in the process-local timezone, so an export/edit/re-import round trip shifts the instant #8485

Found while implementing #8373 (export renders datetime cells in the business timezone); filed rather than fixed there — it is the import side of the same seam and outside that card's declared surface.

What

parseDateCell() in packages/rest/src/import-coerce.ts ends with:

const parsed = new Date(s);
if (Number.isNaN(parsed.getTime())) return undefined;
...
return parsed.toISOString();

A cell like 2026-08-01 06:00:00 carries no offset, so new Date(s) interprets it in the process timezone (ECMAScript: a date-time form with no offset is local time; date-only forms are UTC — which is why the YYYY-MM-DD fast path above it is fine). The import path therefore stores a different instant depending on the deployment host's TZ, and never consults the caller's business timezone, which resolveExecCtx has already resolved on this route.

Measured:

TZ=Asia/Shanghai node -e "console.log(new Date('2026-08-01 06:00:00').toISOString())"
# 2026-07-31T22:00:00.000Z
TZ=UTC           node -e "console.log(new Date('2026-08-01 06:00:00').toISOString())"
# 2026-08-01T06:00:00.000Z

Same file, same tenant, same cell — eight hours apart, decided by a host setting nobody authoring the spreadsheet can see.

Why it matters

Export and import are advertised as inverses (import-coerce.ts opens with "This is the inverse of export-format.ts"), and the export writes a naive YYYY-MM-DD HH:mm:ss cell with no offset. So the natural workflow — export, edit in a spreadsheet, re-import — is only lossless when the host TZ happens to equal the clock the export wrote in:

• Before 【缺陷】数据导出(CSV/XLSX)日期时间列硬编码按 UTC 渲染,与界面时区不一致(@objectstack/rest export-format.ts formatDate) #8373 the export wrote UTC wall time; a host at TZ=Asia/Shanghai re-imported it as +08 and shifted every datetime by 8 hours.
• After 【缺陷】数据导出(CSV/XLSX)日期时间列硬编码按 UTC 渲染,与界面时区不一致(@objectstack/rest export-format.ts formatDate) #8373 the export writes business-tz wall time; the round trip is now correct exactly when TZ equals the business timezone (the reported deployment's case) and wrong by the difference otherwise.

Either way the correctness of the round trip rests on a host env var rather than on the tenant's configured timezone. The defect predates #8373; #8373 changes which deployments happen to be lucky.

Suggested direction

Read the cell in the business timezone: thread ExecutionContext.timezone into the import coercion and, for a naive datetime, interpret the wall clock in that zone. zonedDateStartToUtcMs in packages/core/src/utils/datetime.ts already does the wall-clock-to-instant direction DST-safely (Intl.DateTimeFormat offsets, never hand-rolled arithmetic) and is the natural primitive to generalise from date-only to date-time. Fall back to the current behaviour when no timezone is resolved.

Worth deciding explicitly as part of the fix: whether a cell that does carry an offset (2026-08-01T06:00:00+08:00, or a trailing Z) keeps being honoured as written — it should, and the change should only affect naive cells.

Scope

• packages/rest/src/import-coerce.ts — parseDateCell and its coerceValue caller
• the import route's call sites in packages/rest/src/rest-server.ts / import-prepare.ts
• tests in packages/rest (a cross-day case, not a mid-day one — this shifts rows across month boundaries the same way 【缺陷】数据导出(CSV/XLSX)日期时间列硬编码按 UTC 渲染,与界面时区不一致(@objectstack/rest export-format.ts formatDate) #8373 did)

Related: #8373.

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