diff --git a/src/public/app/widgets/buttons/calendar.ts b/src/public/app/widgets/buttons/calendar.ts
index 87803f6cb..8c0c89f77 100644
--- a/src/public/app/widgets/buttons/calendar.ts
+++ b/src/public/app/widgets/buttons/calendar.ts
@@ -10,10 +10,11 @@ import { Dropdown } from "bootstrap";
 import type { EventData } from "../../components/app_context.js";
 import dayjs, { Dayjs } from "dayjs";
 import utc from "dayjs/plugin/utc.js";
+import isSameOrAfter from "dayjs/plugin/isSameOrAfter.js";
 import "../../../stylesheets/calendar.css";
 
 dayjs.extend(utc);
-
+dayjs.extend(isSameOrAfter);
 const MONTHS = [
     t("calendar.january"),
     t("calendar.febuary"),
@@ -205,59 +206,67 @@ export default class CalendarWidget extends RightDropdownButtonWidget {
 
     getWeekNumber(date: Dayjs): number {
         const year = date.year();
-        const jan1 = date.clone().startOf('year');
-        const jan1Day = jan1.day();
+        const dayOfWeek = (day: number) => (day - this.firstDayOfWeek + 7) % 7;
 
-        let firstWeekStart = jan1.clone();
-
-        let dayOffset;
-        if (jan1Day < this.firstDayOfWeek) {
-            dayOffset = jan1Day + (7 - this.firstDayOfWeek);
-        } else {
-            dayOffset = jan1Day - this.firstDayOfWeek;
-        }
-        firstWeekStart = firstWeekStart.subtract(dayOffset, 'day');
+        // Get first day of the year and adjust to first week start
+        const jan1 = date.clone().year(year).month(0).date(1);
+        const jan1Weekday = jan1.day();
+        const dayOffset = dayOfWeek(jan1Weekday);
+        let firstWeekStart = jan1.clone().subtract(dayOffset, 'day');
 
+        // Adjust based on week rule
         switch (this.weekCalculationOptions.firstWeekType) {
-            // case 0 is default: week containing Jan 1
-            case 1: {
-                let thursday = firstWeekStart.clone();
-                const day = thursday.day();
-                const offset = (4 - day + 7) % 7;
-                thursday = thursday.add(offset, 'day');
+            case 1: { // ISO 8601: first week contains Thursday
+                const thursday = firstWeekStart.clone().add(3, 'day'); // Monday + 3 = Thursday
                 if (thursday.year() < year) {
                     firstWeekStart = firstWeekStart.add(7, 'day');
                 }
                 break;
             }
-            case 2: {
+            case 2: { // minDaysInFirstWeek rule
                 const daysInFirstWeek = 7 - dayOffset;
                 if (daysInFirstWeek < this.weekCalculationOptions.minDaysInFirstWeek) {
                     firstWeekStart = firstWeekStart.add(7, 'day');
                 }
                 break;
             }
+            // default case 0: week containing Jan 1 → already handled
         }
 
-        const diffDays = date.diff(firstWeekStart, 'day') + 1;
+        const diffDays = date.startOf('day').diff(firstWeekStart.startOf('day'), 'day');
         const weekNumber = Math.floor(diffDays / 7) + 1;
 
-        // Check if the week number is less than 0, which means the date is in the previous year
+        // Handle case when date is before first week start → belongs to last week of previous year
         if (weekNumber <= 0) {
-            const prevYearLastWeek = this.getWeekNumber(date.subtract(1, 'year').endOf('year'));
-            return prevYearLastWeek;
+            return this.getWeekNumber(date.subtract(1, 'day'));
         }
 
-        // Check if it's the last week of December
-        if (date.month() === 11) { // December
-            const lastDayOfYear = date.clone().month(11).date(31);
-            const lastWeekStart = lastDayOfYear.subtract((lastDayOfYear.day() - this.firstDayOfWeek + 7) % 7, 'day');
+        // Handle case when date belongs to first week of next year
+        const nextYear = year + 1;
+        const jan1Next = date.clone().year(nextYear).month(0).date(1);
+        const jan1WeekdayNext = jan1Next.day();
+        const offsetNext = dayOfWeek(jan1WeekdayNext);
+        let nextYearWeekStart = jan1Next.clone().subtract(offsetNext, 'day');
 
-            // Use local timezone to avoid date mismatch
-            if (date.isSame(lastWeekStart, 'day')) {
-                const nextYearFirstWeek = this.getWeekNumber(lastDayOfYear.add(1, 'day'));
-                return nextYearFirstWeek;
+        switch (this.weekCalculationOptions.firstWeekType) {
+            case 1: {
+                const thursday = nextYearWeekStart.clone().add(3, 'day');
+                if (thursday.year() < nextYear) {
+                    nextYearWeekStart = nextYearWeekStart.add(7, 'day');
+                }
+                break;
             }
+            case 2: {
+                const daysInFirstWeek = 7 - offsetNext;
+                if (daysInFirstWeek < this.weekCalculationOptions.minDaysInFirstWeek) {
+                    nextYearWeekStart = nextYearWeekStart.add(7, 'day');
+                }
+                break;
+            }
+        }
+
+        if (date.isSameOrAfter(nextYearWeekStart)) {
+            return 1;
         }
 
         return weekNumber;