From 67985719ebc7e93ca8082ddcc29edf8e1852e582 Mon Sep 17 00:00:00 2001
From: Richard Gibson <richard.gibson@gmail.com>
Date: Fri, 19 Sep 2025 15:19:29 -0400
Subject: [PATCH] Polyfill: Speed up largestUnit: month date difference for
 calendars without leap months

Fixes #3153
---
 polyfill/lib/calendar.mjs | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/polyfill/lib/calendar.mjs b/polyfill/lib/calendar.mjs
index 60eca0ecf..031fb2311 100644
--- a/polyfill/lib/calendar.mjs
+++ b/polyfill/lib/calendar.mjs
@@ -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;
@@ -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.