diff --git a/js/src/util/calendar.js b/js/src/util/calendar.js
index dd8a9e831..e04ff7a98 100644
--- a/js/src/util/calendar.js
+++ b/js/src/util/calendar.js
@@ -812,13 +812,13 @@ export const getMonthDetails = (year, month, firstDayOfWeek) => {
 
     if ((index + 1) % 7 === 0) {
       const { weekNumber, year } = getISOWeekNumberAndYear(day.date)
-      const lastWeek = weeks.at(-1)
+      const lastWeek = weeks[weeks.length - 1]
       if (lastWeek) {
         lastWeek.week = { number: weekNumber, year }
       }
     }
 
-    const lastWeek = weeks.at(-1)
+    const lastWeek = weeks[weeks.length - 1]
     if (lastWeek) {
       lastWeek.days.push(day)
     }