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Polyfill: Speed up largestUnit: month date difference for calendars without leap months #3154

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merged 1 commit into from
Sep 19, 2025
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Polyfill: Speed up largestUnit: month date difference for calendars without leap months #3154

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6 changes: 5 additions & 1 deletion polyfill/lib/calendar.mjs
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Original file line number Diff line number Diff line change
Expand Up @@ -1013,7 +1013,7 @@ const nonIsoHelperBase = {
}
const diffYears = calendarTwo.year - calendarOne.year;
const diffDays = calendarTwo.day - calendarOne.day;
if (largestUnit === 'year' && diffYears) {
if (diffYears && (largestUnit === 'year' || !monthCodeInfo[this.id]?.additionalMonths)) {
let diffInYearSign = 0;
if (calendarTwo.monthCode > calendarOne.monthCode) diffInYearSign = 1;
if (calendarTwo.monthCode < calendarOne.monthCode) diffInYearSign = -1;
Expand All @@ -1022,6 +1022,10 @@ const nonIsoHelperBase = {
years = isOneFurtherInYear ? diffYears - sign : diffYears;
}
const yearsAdded = years ? this.addCalendar(calendarOne, { years }, 'constrain', cache) : calendarOne;
if (years && largestUnit === 'month') {
months += years * 12;
years = 0;
}
// Now we have less than one year remaining. Add one month at a time
// until we go over the target, then back up one month and calculate
// remaining days and weeks.
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