round years using calendar years only

Fixes #270

Author: keithamus

src/duration.ts

@@ -151,33 +151,45 @@ export function roundToSingleUnit(duration: Duration, {relativeTo = Date.now()}:
   if (!days && hours >= 21) days += Math.round(hours / 24)
   if (days || weeks || months || years) hours = 0
 
+  // Resolve calendar dates
   const currentYear = relativeTo.getFullYear()
   let currentMonth = relativeTo.getMonth()
   const currentDate = relativeTo.getDate()
-  if (days >= 27 || (years + months && days)) {
-    relativeTo.setDate(currentDate + days * sign)
-    months += Math.abs(
-      relativeTo.getFullYear() >= currentYear
-        ? relativeTo.getMonth() - currentMonth
-        : relativeTo.getMonth() - currentMonth - 12,
-    )
-    if (months) {
-      days = 0
+  if (days >= 27 || years + months + days) {
+    const newDate = new Date(relativeTo)
+    newDate.setFullYear(currentYear + years * sign)
+    newDate.setMonth(currentMonth + months * sign)
+    newDate.setDate(currentDate + 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 monthsDiff = Math.abs(yearDiff * 12 + monthDiff)
+    if (daysDiff < 27) {
+      if (days >= 6) {
+        weeks += Math.round(days / 7)
+        days = 0
+      } else {
+        days = daysDiff
+      }
+      months = years = 0
+    } else if (monthsDiff < 11) {
+      months = monthsDiff
+      years = 0
+    } else {
+      months = 0
+      years = yearDiff * sign
     }
+    if (months || years) days = 0
     currentMonth = relativeTo.getMonth()
   }
-
-  if (days >= 6) weeks += Math.round(days / 7)
-  if (weeks || months || years) days = 0
+  if (years) months = 0
 
   if (weeks >= 4) months += Math.round(weeks / 4)
   if (months || years) weeks = 0
-
-  if (months >= 11 || (years && months)) {
-    relativeTo.setMonth(relativeTo.getMonth() + months * sign)
-    years += Math.abs(currentYear - relativeTo.getFullYear())
+  if (days && weeks && !months && !years) {
+    weeks += Math.round(days / 7)
+    days = 0
   }
-  if (years) months = 0
 
   return new Duration(
     years * sign,

test/duration.ts

@@ -293,6 +293,7 @@ suite('duration', function () {
       ['P9M20DT25H', 'P10M', {relativeTo: new Date('2023-01-12T00:00:00Z')}],
       ['P11M', 'P1Y', {relativeTo: new Date('2022-11-01T00:00:00Z')}],
       ['-P11M', '-P1Y', {relativeTo: new Date('2022-11-01T00:00:00Z')}],
+      ['-P11M15D', '-P1Y', {relativeTo: new Date('2024-01-06T00:00:00')}],
       ['P1Y4D', 'P1Y', {relativeTo: new Date('2022-11-01T00:00:00Z')}],
       ['P1Y5M13D', 'P1Y', {relativeTo: new Date('2023-01-01T00:00:00Z')}],
       ['P1Y5M15D', 'P1Y', {relativeTo: new Date('2023-01-01T00:00:00Z')}],
@@ -308,7 +309,12 @@ suite('duration', function () {
           relativeTo: new Date('2022-01-01T00:00:00Z'),
         },
       ],
-      ['-P27D', '-P1M', {relativeTo: new Date('2023-02-28T00:00:00Z')}],
+      ['-P27D', '-P27D', {relativeTo: new Date('2023-02-28T00:00:00Z')}],
+      ['-P27D', '-P1M', {relativeTo: new Date('2023-02-27T00:00:00Z')}],
+      ['P1Y2M1D', 'P2Y', {relativeTo: new Date('2022-12-31T12:00:00.000Z')}],
+      ['-P1Y8D', '-P1Y', {relativeTo: new Date('2024-01-11T12:00:00.000Z')}],
+      ['-P1Y7DT19H43M19S', '-P1Y', {relativeTo: new Date('2024-01-11T12:00:00.000Z')}],
+      ['-P1Y11D', '-P2Y', {relativeTo: new Date('2024-01-11T12:00:00.000Z')}],
     ])
     for (const [input, expected, opts] of roundTests) {
       test(`roundToSingleUnit(${input}) === ${expected}`, () => {

test/relative-time.js

@@ -482,7 +482,7 @@ suite('relative-time', function () {
       time.setAttribute('datetime', datetime)
       time.setAttribute('format', 'micro')
       await Promise.resolve()
-      assert.equal(time.shadowRoot.textContent, '10y')
+      assert.equal(time.shadowRoot.textContent, '11y')
     })
 
     test('micro formats future times', async () => {
@@ -2416,14 +2416,14 @@ suite('relative-time', function () {
         datetime: '2024-03-01T12:00:00.000Z',
         tense: 'future',
         format: 'auto',
-        expected: 'in 3 years',
+        expected: 'in 2 years',
       },
       {
         reference: '2022-12-31T12:00:00.000Z',
         datetime: '2024-03-01T12:00:00.000Z',
         tense: 'future',
         format: 'micro',
-        expected: '3y',
+        expected: '2y',
       },
       {
         reference: '2021-04-24T12:00:00.000Z',
@@ -2432,6 +2432,13 @@ suite('relative-time', function () {
         format: 'micro',
         expected: '2y',
       },
+      {
+        reference: '2024-01-04T12:00:00.000Z',
+        datetime: '2020-02-16T16:16:41.000Z',
+        tense: 'past',
+        format: 'auto',
+        expected: '4 years ago',
+      },
     ])
 
     for (const {