correctly count month durations relative to the end of longer months

Author: lilyinstarlight

src/duration.ts

@@ -153,16 +153,23 @@ export function roundToSingleUnit(duration: Duration, {relativeTo = Date.now()}:
 
   // Resolve calendar dates
   const currentYear = relativeTo.getFullYear()
-  let currentMonth = relativeTo.getMonth()
+  const currentMonth = relativeTo.getMonth()
   const currentDate = relativeTo.getDate()
   if (days >= 27 || years + months + days) {
+    const newMonthDate = new Date(relativeTo)
+    newMonthDate.setDate(1)
+    newMonthDate.setMonth(currentMonth + months * sign + 1)
+    newMonthDate.setDate(0)
+    const monthDateCorrection = Math.max(0, currentDate - newMonthDate.getDate())
+
     const newDate = new Date(relativeTo)
     newDate.setFullYear(currentYear + years * sign)
+    newDate.setDate(currentDate - monthDateCorrection)
     newDate.setMonth(currentMonth + months * sign)
-    newDate.setDate(currentDate + days * sign)
+    newDate.setDate(currentDate - monthDateCorrection + days * sign)
     const yearDiff = newDate.getFullYear() - relativeTo.getFullYear()
     const monthDiff = newDate.getMonth() - relativeTo.getMonth()
-    const daysDiff = Math.abs(Math.round((Number(newDate) - Number(relativeTo)) / 86400000))
+    const daysDiff = Math.abs(Math.round((Number(newDate) - Number(relativeTo)) / 86400000)) + monthDateCorrection
     const monthsDiff = Math.abs(yearDiff * 12 + monthDiff)
     if (daysDiff < 27) {
       if (days >= 6) {
@@ -180,7 +187,6 @@ export function roundToSingleUnit(duration: Duration, {relativeTo = Date.now()}:
       years = yearDiff * sign
     }
     if (months || years) days = 0
-    currentMonth = relativeTo.getMonth()
   }
   if (years) months = 0
 

test/duration.ts

@@ -367,6 +367,9 @@ suite('duration', function () {
       ['-P55D', [-1, 'month'], {relativeTo: '2023-02-27T22:22:57Z'}],
       ['-P65D', [-3, 'month'], {relativeTo: '2023-02-28T22:22:57Z'}],
       ['-P75D', [-3, 'month'], {relativeTo: '2023-03-09T22:22:57Z'}],
+      ['P1M', [1, 'month'], {relativeTo: '2024-05-31T00:00:00Z'}],
+      ['-P1M', [-1, 'month'], {relativeTo: '2024-05-31T00:00:00Z'}],
+      ['-P3M', [-3, 'month'], {relativeTo: '2023-05-30T00:00:00Z'}],
       [
         'P8M',
         [8, 'month'],
@@ -396,6 +399,8 @@ suite('duration', function () {
         },
       ],
       ['P1M1D', [1, 'month'], {relativeTo: new Date('2022-12-01T00:00:00Z')}],
+      ['P1M1D', [2, 'month'], {relativeTo: new Date('2023-01-31T00:00:00Z')}],
+      ['P1M30D', [2, 'month'], {relativeTo: new Date('2023-01-31T00:00:00Z')}],
       [
         'P9M20DT25H',
         [9, 'month'],
@@ -478,14 +483,14 @@ suite('duration', function () {
       ],
     ])
     for (const [input, [val, unit], opts] of relativeTests) {
-      test(`getRelativeTimeUnit(${input}) === [${val}, ${unit}]`, () => {
+      test(`getRelativeTimeUnit(${input}${opts ? `, ${JSON.stringify(opts)}` : ''}) === [${val}, ${unit}]`, () => {
         assert.deepEqual(
           getRelativeTimeUnit(Duration.from(input), opts || {relativeTo: new Date('2023-07-01T00:00:00')}),
           [val, unit],
         )
       })
       if (opts?.relativeTo) continue
-      test(`getRelativeTimeUnit(-${input}) === [-${val}, ${unit}]`, () => {
+      test(`getRelativeTimeUnit(-${input}${opts ? `, ${JSON.stringify(opts)}` : ''}) === [-${val}, ${unit}]`, () => {
         assert.deepEqual(
           getRelativeTimeUnit(Duration.from(`-${input}`), opts || {relativeTo: new Date('2023-07-01T00:00:00')}),
           [-val, unit],

test/relative-time.js

@@ -482,6 +482,24 @@ suite('relative-time', function () {
       assert.equal(time.shadowRoot.textContent, '4 months ago')
     })
 
+    test('rewrites from last few days of month to smaller last month', async () => {
+      freezeTime(new Date(2024, 4, 31))
+      const time = document.createElement('relative-time')
+      time.setAttribute('tense', 'past')
+      time.setAttribute('datetime', '2024-04-30T00:00:00Z')
+      await Promise.resolve()
+      assert.equal(time.shadowRoot.textContent, 'last month')
+    })
+
+    test('rewrites from last few days of month to smaller previous month', async () => {
+      freezeTime(new Date(2024, 4, 31))
+      const time = document.createElement('relative-time')
+      time.setAttribute('tense', 'past')
+      time.setAttribute('datetime', '2024-02-29T00:00:00Z')
+      await Promise.resolve()
+      assert.equal(time.shadowRoot.textContent, '3 months ago')
+    })
+
     test('micro formats years', async () => {
       const datetime = new Date()
       datetime.setFullYear(datetime.getFullYear() - 10)