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Correctly round same-month day durations #361

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Correctly round same-month day durations #361

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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.
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@thazhemadam
thazhemadam committed Jun 29, 2026
commit dda2ab09195553ba3181bcab37a35259aa213ed4
6 changes: 4 additions & 2 deletions src/duration.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -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
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7 changes: 5 additions & 2 deletions test/duration.ts
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Expand Up @@ -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')}],
Expand Down Expand Up @@ -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'}],
Expand Down
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