js-on-sx: Date.UTC and new Date propagate NaN/Infinity args

plans/js-on-sx.md

@@ -158,6 +158,8 @@ Each item: implement → tests → update progress. Mark `[x]` when tests green.
 
 Append-only record of completed iterations. Loop writes one line per iteration: date, what was done, test count delta.
 
+- 2026-05-10 — **`Date.UTC` / `new Date(...)` propagate NaN/±Infinity arguments and return NaN.** `Date.UTC()` (no args) returned 0 instead of NaN; `Date.UTC(NaN, ...)` did the math and produced bogus ms; `new Date(year, NaN)` constructed a normal Date instead of an invalid one. Added `js-date-args-have-nan?` (also detects ±Infinity and propagates from rationals) used by both `Date.UTC` and the multi-arg constructor branch; UTC now returns NaN on no-arg / any-NaN-arg / out-of-range result, and `new Date(args)` stores NaN in `__date_value__` when any arg is NaN. Also fixed `js-date-from-one(undefined)` to return NaN. Result: built-ins/Date/UTC 6/16 → 10/16 (+4). Date 17/30 → 26/30 (timeouts dropped from 12 → 4 because invalid Dates now short-circuit). conformance.sh: 148/148.
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 - 2026-05-10 — **Real `Date` construction + getters via Howard-Hinnant civil-day arithmetic.** `js-date-from-parts` now computes a true ms-since-epoch from `(year, month, day, hour, min, sec, ms)` via `js-date-civil-to-days` (the inverse of last iteration's `days-to-ymd`), with the legacy 2-digit-year coercion (0..99 → 1900+y). `getFullYear/Month/Date/Day/Hours/Minutes/Seconds/Milliseconds` (UTC + non-UTC) all share a new `js-date-getter`: TypeErrors on non-Date this, returns NaN on invalid time, otherwise decomposes ms into y/m/d/h/m/s/ms/dow. Plus added `Date.prototype.constructor = Date` (was missing). Result: each of the 8 Date getter categories went 2/6 → 5/6 (+3 each, +24 total). Date toISOString 11/16 → 13/16. Some Date construction-loop tests now exceed the 15s per-test timeout — the new civil math is heavier than the old (year-1970)*ms-per-year approximation, but correctness wins. conformance.sh: 148/148.
 
 - 2026-05-10 — **`Date.prototype.toISOString` produces real `YYYY-MM-DDTHH:mm:ss.sssZ` and validates input.** Old `js-date-iso` only computed the year and hardcoded the rest as `01-01T00:00:00.000Z`. Added: (1) TypeError when this isn't a Date (no `__js_is_date__` slot); (2) RangeError when ms is NaN, undefined, or |ms| > 8.64e15; (3) full date breakdown via Howard-Hinnant `days_to_civil` algorithm (`js-date-days-to-ymd`) → year/month/day, plus modular hours/min/sec/ms; (4) extended-year format `±YYYYYY` for years outside 0..9999. Result: built-ins/Date/prototype/toISOString 7/16 → 11/16 (+4). Date 21/30. conformance.sh: 148/148.