fix: keep same-month day durations monotonic

Relative date labels around the month boundary can currently move
backward in precision: a 25-day-old timestamp may render as "last month"
while a 27-day-old timestamp renders as "27 days ago". In sorted date
lists this makes newer items look older than later rows, even when the
underlying timestamps are ordered correctly.

Keep same-calendar-month day spans in days instead of promoting them
through the intermediate four-week representation to one month. Preserve
month rounding once the compared dates cross a calendar-month boundary.

Add regression coverage for the 25/26-day same-month cases so the labels
remain monotonic around the month boundary.

Author: thazhemadam

src/duration.ts

@@ -181,8 +181,10 @@ export function roundToSingleUnit(duration: Duration, {relativeTo = Date.now()}:
     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) {
-        weeks += Math.round(days / 7)
+      const roundedWeeks = Math.round(days / 7)
+      // Keep same-calendar-month spans as days instead of rounding 4 weeks to a month.
+      if (days >= 6 && (roundedWeeks < 4 || monthsDiff > 0)) {
+        weeks += roundedWeeks
         days = 0
       } else {
         days = daysDiff

test/duration.ts

@@ -259,9 +259,9 @@ suite('duration', function () {
       ['-P21D', '-P3W', {relativeTo: new Date('2023-07-01T00:00:00')}],
       ['P24D', 'P3W', {relativeTo: new Date('2023-07-01T00:00:00')}],
       ['-P24D', '-P3W', {relativeTo: new Date('2023-07-01T00:00:00')}],
-      ['P24DT25H', 'P1M', {relativeTo: new Date('2023-07-01T00:00:00')}],
+      ['P24DT25H', 'P25D', {relativeTo: new Date('2023-07-01T00:00:00')}],
       ['-P24DT25H', '-P1M', {relativeTo: new Date('2023-07-01T00:00:00')}],
-      ['P25D', 'P1M', {relativeTo: new Date('2023-07-01T00:00:00')}],
+      ['P25D', 'P25D', {relativeTo: new Date('2023-07-01T00:00:00')}],
       ['-P25D', '-P1M', {relativeTo: new Date('2023-07-01T00:00:00')}],
       ['P1M1D', 'P1M', {relativeTo: new Date('2023-07-02T00:00:00')}],
       ['-P1M1D', '-P1M', {relativeTo: new Date('2023-07-02T00:00:00')}],
@@ -365,6 +365,9 @@ suite('duration', function () {
       ['-P24D', [-3, 'week'], {relativeTo: '2023-01-01T00:00:00Z'}],
       ['P24DT25H', [1, 'month'], {relativeTo: '2023-01-15T00:00:00Z'}],
       ['P25D', [1, 'month'], {relativeTo: '2023-01-15T00:00:00Z'}],
+      ['P25D', [25, 'day'], {relativeTo: '2023-07-01T00:00:00Z'}],
+      ['-P25D', [-25, 'day'], {relativeTo: '2026-06-29T00:00:00Z'}],
+      ['-P26D', [-26, 'day'], {relativeTo: '2026-06-29T00:00:00Z'}],
       ['-P35D', [-1, 'month'], {relativeTo: '2023-02-07T22:22:57Z'}],
       ['-P45D', [-1, 'month'], {relativeTo: '2023-02-17T22:22:57Z'}],
       ['-P55D', [-1, 'month'], {relativeTo: '2023-02-27T22:22:57Z'}],