[JS] DateMilliseconds is not always decoded correcly

[Fil]

Doing a round-trip with a JavaScript Date in apache-arrow/js sometimes results in wrong values:

import * as Arrow from "apache-arrow";
const date = new Date(1950, 1, 0);
const data = [{date}]
console.log("true date", date, +date);
const arrow = Arrow.tableFromJSON(data);
const [lo, hi] = arrow.batches[0].data.children[0].values;
console.log("encoded bytes", hi * (2**32) + lo);
for (const d of arrow) console.log("decoded date", d.date, +d.date)

this prints:

❯ node test-arrow.js
true date 1950-01-30T23:00:00.000Z -628563600000
encoded bytes -628563600000
decoded date 1950-03-21T16:02:47.296Z -624268632704

As you can see there is a difference of 45 days. If I go and modify factories.ts and change dtypes.DateMillisecond to dtypes.Float64 (line 122), the decoding returns the correct number.

version information:

apache-arrow@^15.0.2:
  resolved "https://registry.yarnpkg.com/apache-arrow/-/apache-arrow-15.0.2.tgz#d87c6447d64d6fab34aa70119362680b6617ce63"

❯ node -v
v20.11.1

macos sonoma 14.3.1 (23D60)

The results are correct for dates after epoch, but testing with new Date(1969, 11, 31); exhibits the problem.

Component(s)

JavaScript

[trxcllnt]

Looks like these lines should be Math.floor() instead of Math.trunc().

[Fil]

Yes that works for me! thanks

[domoritz]

Fix in #40725 and I added a simple test.

[domoritz]

Fixed in #40725