#1165: Fixing and Testing "Duration.diff returns negative number"

[JoaoAugustoPansani]

On the file ./scripts/src/impl/diff.js there's a function dayDiff:

function dayDiff(earlier, later) {
  const utcDayStart = (dt) => dt.toUTC(0, { keepLocalTime: true }).startOf("day").valueOf(),
    ms = utcDayStart(later) - utcDayStart(earlier);
  return Math.floor(Duration.fromMillis(ms).as("days"));
}

What I did for solving this issue, was delete the second argument inside .toUTC:

function dayDiff(earlier, later) {
  const utcDayStart = (dt) => dt.toUTC(0).startOf("day").valueOf(),
    ms = utcDayStart(later) - utcDayStart(earlier);
  return Math.floor(Duration.fromMillis(ms).as("days"));
}

Then I wrote a test inside ./scripts/test/datetime/diff.test.js:

test("DateTime#diff works when date has a zone object", () => {
  const start = DateTime.fromISO("2022-05-05T23:00", { zone: "UTC" });
  const end = DateTime.fromISO("2022-05-10T00:00", { zone: "Europe/Madrid" });

  const diff = end.diff(start, ["days", "hours", "minutes"]).toObject();
  expect(diff.days).toBe(3);
  expect(diff.hours).toBe(23);
  expect(diff.minutes).toBe(0);
});

[linux-foundation-easycla[bot]]

The committers listed above are authorized under a signed CLA.

• :white_check_mark: login: JoaoAugustoPansani / name: João Augusto Mendonça Pansani (c55a8bed649819a76549372c8071f6966dad05eb, 65124dab34162bbb7299e0651877b6f7bc0f4528)

[icambron]

Sorry for the slow response. I don't think this fix is correct, though. That second argument ensures that diff compares local times, not absolute ones. You can see the problem by trying these dates instead:

const start = DateTime.fromISO("2022-05-06T00:00", { zone: "UTC" });
const end = DateTime.fromISO("2022-05-10T00:00", { zone: "Europe/Madrid" });

dayDiff(start, end) // returns 3, not 4

[JoaoAugustoPansani]

No problem @icambron, that's fine. I'll work on that, thanks for the response!

[omasback]

Sorry I've only briefly looked at this but why not just convert both datetimes toMillis() and subtract? Then convert millis to Duration and shiftTo() back to requested units?