diff --git a/code/frontend/src/app/shared/utils/chart-window.util.spec.ts b/code/frontend/src/app/shared/utils/chart-window.util.spec.ts
index f4c21d08..3a040c53 100644
--- a/code/frontend/src/app/shared/utils/chart-window.util.spec.ts
+++ b/code/frontend/src/app/shared/utils/chart-window.util.spec.ts
@@ -45,6 +45,17 @@ describe('chart-window.util', () => {
   });
 
   describe('formatBucketDate', () => {
+    afterEach(() => {
+      vi.restoreAllMocks();
+    });
+
+    const forceTwelveHourLocale = () => {
+      const original = Date.prototype.toLocaleTimeString;
+      vi.spyOn(Date.prototype, 'toLocaleTimeString').mockImplementation(function (this: Date, _locale, options) {
+        return original.call(this, 'en-US', options);
+      });
+    };
+
     it('keeps the calendar day of the bucket regardless of the local timezone', () => {
       expect(formatBucketDate('2026-03-15T23:00:00Z', 168)).toContain('15');
     });
@@ -61,6 +72,18 @@ describe('chart-window.util', () => {
       expect(formatted).toMatch(/\d/);
       expect(formatted).not.toBe(formatBucketDate('2026-03-15T23:00:00Z', 168));
     });
+
+    it('formats 24h windows with a 24-hour clock', () => {
+      forceTwelveHourLocale();
+
+      expect(formatBucketDate(new Date(2026, 2, 15, 13, 0).toISOString(), 24)).toBe('13:00');
+    });
+
+    it('formats midnight as 00 rather than 24', () => {
+      forceTwelveHourLocale();
+
+      expect(formatBucketDate(new Date(2026, 2, 15, 0, 0).toISOString(), 24)).toBe('00:00');
+    });
   });
 
   describe('getChartDuration', () => {
diff --git a/code/frontend/src/app/shared/utils/chart-window.util.ts b/code/frontend/src/app/shared/utils/chart-window.util.ts
index 8037fba4..8cd69e15 100644
--- a/code/frontend/src/app/shared/utils/chart-window.util.ts
+++ b/code/frontend/src/app/shared/utils/chart-window.util.ts
@@ -28,7 +28,7 @@ export function getChartDuration(): number {
 
 export function formatBucketDate(date: string, hours: number): string {
   if (hours <= 24) {
-    return new Date(date).toLocaleTimeString(undefined, { hour: '2-digit', minute: '2-digit' });
+    return new Date(date).toLocaleTimeString(undefined, { hour: '2-digit', minute: '2-digit', hourCycle: 'h23' });
   }
   if (hours >= 8760) {
     return new Date(`${date.slice(0, 10)}T00:00:00`).toLocaleDateString(undefined, { month: 'short', year: '2-digit' });