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fix: roll Feb 29 anniversary anchors over to Mar 1 in monthDiff (#1000) #3209

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fix: roll Feb 29 anniversary anchors over to Mar 1 in monthDiff (#1000) #3209

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11 changes: 10 additions & 1 deletion src/utils.js
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Original file line number Diff line number Diff line change
Expand Up @@ -18,7 +18,16 @@ const monthDiff = (a, b) => {
// function from moment.js in order to keep the same result
if (a.date() < b.date()) return -monthDiff(b, a)
const wholeMonthDiff = ((b.year() - a.year()) * 12) + (b.month() - a.month())
const anchor = a.clone().add(wholeMonthDiff, C.M)
let anchor = a.clone().add(wholeMonthDiff, C.M)
// #1000 A Feb 29 anniversary does not exist in a non-leap year: `add`
// clamps it to Feb 28, which makes `diff` report a full year one day
// early (a person born on 2016-02-29 turns 1 on 2017-03-01, not on
// 2017-02-28). Roll such anchors over to Mar 1, following the common
// legal convention for leap-day anniversaries.
if (a.month() === 1 && a.date() === 29
&& anchor.month() === 1 && anchor.date() === 28) {
anchor = anchor.add(1, C.D)
}
const c = b - anchor < 0
const anchor2 = a.clone().add(wholeMonthDiff + (c ? -1 : 1), C.M)
return +(-(wholeMonthDiff + ((b - anchor) / (c ? (anchor - anchor2) :
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13 changes: 13 additions & 0 deletions test/display.test.js
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Expand Up @@ -218,6 +218,19 @@ describe('Difference', () => {
expect(dayjs('2018-01-01').diff(dayjs('2018-01-01'), 'month')).toEqual(0)
})

it('MonthDiff for leap-day anniversaries (#1000)', () => {
// a leap-day anniversary rolls over to Mar 1 in non-leap years
expect(dayjs('2017-02-28').diff(dayjs('2016-02-29'), 'year')).toEqual(0)
expect(dayjs('2017-03-01').diff(dayjs('2016-02-29'), 'year')).toEqual(1)
expect(dayjs('2017-02-28').diff(dayjs('2016-02-29'), 'month')).toEqual(11)
expect(dayjs('2017-03-01').diff(dayjs('2016-02-29'), 'month')).toEqual(12)
// leap-year anniversaries are unaffected
expect(dayjs('2020-02-28').diff(dayjs('2016-02-29'), 'year')).toEqual(3)
expect(dayjs('2020-02-29').diff(dayjs('2016-02-29'), 'year')).toEqual(4)
// non leap-day boundaries keep clamping to end of month
expect(dayjs('2016-02-29').diff(dayjs('2016-01-31'), 'month')).toEqual(1)
})

it('undefined edge case', () => {
expect(dayjs().diff(undefined, 'seconds')).toBeDefined()
})
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