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fix: clamp day-of-month when adding years/months (calendar overflow) #41

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fix: clamp day-of-month when adding years/months (calendar overflow) #41

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7 changes: 6 additions & 1 deletion lib/index.js
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Original file line number Diff line number Diff line change
Expand Up @@ -67,9 +67,14 @@ var end = function (durationInput, startDate) {
// Create two equal timestamps, add duration to 'then' and return time difference
var timestamp = startDate.getTime();
var then = new Date(timestamp);
// Add years/months on the 1st, then clamp the day to the resulting month so
// that e.g. Jan 31 + P1M is Feb 28, not an overflow into March
var dayOfMonth = then.getDate();
then.setDate(1);
then.setFullYear(then.getFullYear() + duration.years);
then.setMonth(then.getMonth() + duration.months);
then.setDate(then.getDate() + duration.days);
var daysInMonth = new Date(then.getFullYear(), then.getMonth() + 1, 0).getDate();
then.setDate(Math.min(dayOfMonth, daysInMonth) + duration.days);
// set time as milliseconds to get fractions working for minutes/hours
var hoursInMs = duration.hours * 3600 * 1000;
var minutesInMs = duration.minutes * 60 * 1000;
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11 changes: 10 additions & 1 deletion src/index.ts
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Expand Up @@ -89,9 +89,18 @@ export const end = (
const timestamp = startDate.getTime();
const then = new Date(timestamp);

// Add years/months on the 1st, then clamp the day to the resulting month so
// that e.g. Jan 31 + P1M is Feb 28, not an overflow into March
const dayOfMonth = then.getDate();
then.setDate(1);
then.setFullYear(then.getFullYear() + duration.years);
then.setMonth(then.getMonth() + duration.months);
then.setDate(then.getDate() + duration.days);
const daysInMonth = new Date(
then.getFullYear(),
then.getMonth() + 1,
0,
).getDate();
then.setDate(Math.min(dayOfMonth, daysInMonth) + duration.days);
// set time as milliseconds to get fractions working for minutes/hours
const hoursInMs = duration.hours * 3600 * 1000;
const minutesInMs = duration.minutes * 60 * 1000;
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16 changes: 16 additions & 0 deletions test/iso8601-tests.mjs
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Expand Up @@ -133,6 +133,22 @@ test("toSeconds: with supplied start date", () => {
assert.equal(durFromFeb, expectedFebDuration);
});

[
// Jan 31 + 1 month must clamp to Feb 28, not overflow into March
["P1M", new Date(2021, 0, 31), new Date(2021, 1, 28)],
// Feb 29 + 1 year must clamp to Feb 28 in a non-leap year
["P1Y", new Date(2020, 1, 29), new Date(2021, 1, 28)],
// overflow is clamped before days are added: Feb 28 + 1 day = Mar 1
["P1M1D", new Date(2021, 0, 31), new Date(2021, 2, 1)],
].forEach(([value, start, expectedEnd]) => {
test(`toSeconds: clamps calendar overflow (${value})`, () => {
assert.equal(
toSeconds(parse(value), start),
(expectedEnd.getTime() - start.getTime()) / 1000,
);
});
});

test("usage example test", () => {
// Arrange
const jsonString = JSON.stringify({
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