Merge pull request #461 from spokodev/fix-leap-month-add

fix(add): clamp month arithmetic to the leap-aware month length

Author: spencermountain

src/methods/add.js

@@ -1,7 +1,6 @@
 import walkTo from './set/walk.js'
 import ms from '../data/milliseconds.js'
-import monthLength from '../data/monthLengths.js'
-import { months, daysBack, days } from './set/_model.js'
+import { months, daysBack, days, getMonthLength } from './set/_model.js'
 import { normalize } from '../fns.js'
 // this logic is a bit of a mess,
 // but briefly:
@@ -153,7 +152,8 @@ const addMethods = (SpaceTime) => {
     }
     //keep current date, unless the month doesn't have it.
     if (keepDate[unit]) {
-      const max = monthLength[want.month]
+      const year = want.year !== undefined ? want.year : old.year()
+      const max = getMonthLength(want.month, year)
       want.date = old.date()
       if (want.date > max) {
         want.date = max

src/methods/set/_model.js

@@ -1,7 +1,7 @@
 import monthLength from '../../data/monthLengths.js'
 import { isLeapYear } from '../../fns.js'
 
-const getMonthLength = function (month, year) {
+export const getMonthLength = function (month, year) {
   if (month === 1 && isLeapYear(year)) {
     return 29
   }

test/add.test.js

@@ -159,3 +159,12 @@ test('year-tricky', (t) => {
   t.equal(b.year(), 2015, 'year-last')
   t.end()
 })
+
+test('add/subtract month clamps to the leap-aware month length', (t) => {
+  t.equal(spacetime('2020-01-31').add(1, 'month').format('iso-short'), '2020-02-29', 'jan 31 + 1mo -> feb 29 (leap)')
+  t.equal(spacetime('2024-01-31').add(1, 'month').format('iso-short'), '2024-02-29', 'jan 31 + 1mo -> feb 29 (leap)')
+  t.equal(spacetime('2000-01-31').add(1, 'month').format('iso-short'), '2000-02-29', 'jan 31 + 1mo -> feb 29 (400-year leap)')
+  t.equal(spacetime('2021-01-31').add(1, 'month').format('iso-short'), '2021-02-28', 'jan 31 + 1mo -> feb 28 (common year)')
+  t.equal(spacetime('2020-03-31').subtract(1, 'month').format('iso-short'), '2020-02-29', 'mar 31 - 1mo -> feb 29 (leap)')
+  t.end()
+})