diff --git a/utils/dateHelpers.test.ts b/utils/dateHelpers.test.ts
index 95efa1ffe..86a20b48a 100644
--- a/utils/dateHelpers.test.ts
+++ b/utils/dateHelpers.test.ts
@@ -1,5 +1,5 @@
 import { describe, it, expect } from 'vitest';
-import { getAuthorLocalHour, getViewerLocalHour } from './dateHelpers';
+import { getAuthorLocalHour, getViewerLocalHour, processCommitTimestamps } from './dateHelpers';
 
 describe('dateHelpers', () => {
   describe('getAuthorLocalHour', () => {
@@ -24,6 +24,15 @@ describe('dateHelpers', () => {
       expect(getAuthorLocalHour('invalid-date')).toBe(0);
       expect(getAuthorLocalHour('')).toBe(0);
     });
+
+    it('validates ISO 8601 date format before substring extraction', () => {
+      // Valid ISO format extracts hour from substring
+      expect(getAuthorLocalHour('2024-03-10T15:30:00Z')).toBe(15);
+      // Non-ISO format strings fall back to Date parsing
+      const hour = getAuthorLocalHour('March 10, 2024 15:30');
+      expect(hour).toBeGreaterThanOrEqual(0);
+      expect(hour).toBeLessThanOrEqual(23);
+    });
   });
 
   describe('getViewerLocalHour', () => {
@@ -38,4 +47,62 @@ describe('dateHelpers', () => {
       expect(getViewerLocalHour('')).toBe(0);
     });
   });
+
+  describe('processCommitTimestamps', () => {
+    it('returns zero metrics for an empty array', () => {
+      const result = processCommitTimestamps([]);
+      expect(result).toEqual({ morning: 0, afternoon: 0, evening: 0, night: 0 });
+    });
+
+    it('returns zero metrics when all dates are invalid', () => {
+      const result = processCommitTimestamps(['invalid-date', 'not-a-date', '']);
+      expect(result).toEqual({ morning: 0, afternoon: 0, evening: 0, night: 0 });
+    });
+
+    it('returns zero metrics when all dates are null or undefined', () => {
+      const result = processCommitTimestamps([null as unknown as string, undefined as unknown as string]);
+      expect(result).toEqual({ morning: 0, afternoon: 0, evening: 0, night: 0 });
+    });
+
+    it('returns zero metrics for an array containing only Invalid Date strings', () => {
+      const result = processCommitTimestamps(['2024-13-99T25:99:00Z', 'hello world']);
+      expect(result).toEqual({ morning: 0, afternoon: 0, evening: 0, night: 0 });
+    });
+
+    it('counts valid morning commits correctly', () => {
+      const result = processCommitTimestamps(['2024-03-10T09:00:00Z', '2024-03-10T11:30:00Z']);
+      expect(result.morning).toBe(2);
+      expect(result.afternoon).toBe(0);
+      expect(result.evening).toBe(0);
+      expect(result.night).toBe(0);
+    });
+
+    it('counts valid afternoon commits correctly', () => {
+      const result = processCommitTimestamps(['2024-03-10T12:00:00Z', '2024-03-10T17:59:00Z']);
+      expect(result.morning).toBe(0);
+      expect(result.afternoon).toBe(2);
+    });
+
+    it('counts valid evening commits correctly', () => {
+      const result = processCommitTimestamps(['2024-03-10T18:00:00Z', '2024-03-10T23:59:00Z']);
+      expect(result.evening).toBe(2);
+    });
+
+    it('counts valid night commits correctly', () => {
+      const result = processCommitTimestamps(['2024-03-10T00:00:00Z', '2024-03-10T05:59:00Z']);
+      expect(result.night).toBe(2);
+    });
+
+    it('ignores invalid dates while counting valid ones', () => {
+      const result = processCommitTimestamps([
+        '2024-03-10T09:00:00Z',
+        'invalid-date',
+        '2024-03-10T14:00:00Z',
+      ]);
+      expect(result.morning).toBe(1);
+      expect(result.afternoon).toBe(1);
+      expect(result.night).toBe(0);
+      expect(result.evening).toBe(0);
+    });
+  });
 });
diff --git a/utils/dateHelpers.ts b/utils/dateHelpers.ts
index b1d7a25e4..b4db3be4d 100644
--- a/utils/dateHelpers.ts
+++ b/utils/dateHelpers.ts
@@ -11,6 +11,7 @@ export function processCommitTimestamps(commitDates: string[] | Date[]): TimeOfD
   commitDates.forEach((dateString) => {
     if (!dateString) return;
     const date = new Date(dateString);
+    if (isNaN(date.getTime())) return;
     const hour = date.getHours();
 
     if (hour >= 6 && hour < 12) {
@@ -34,6 +35,12 @@ export function processCommitTimestamps(commitDates: string[] | Date[]): TimeOfD
 export function getAuthorLocalHour(isoDate: string): number {
   if (!isoDate || typeof isoDate !== 'string') return 0;
 
+  // Validate ISO 8601 date format: YYYY-MM-DDTHH:MM:SS...
+  if (!/^\d{4}-\d{2}-\d{2}T/.test(isoDate)) {
+    const parsed = new Date(isoDate);
+    return isNaN(parsed.getTime()) ? 0 : parsed.getHours();
+  }
+
   if (isoDate.length >= 13) {
     const hourStr = isoDate.substring(11, 13);
     const hour = parseInt(hourStr, 10);