diff --git a/src/lib/__tests__/yearInReviewUtils.test.ts b/src/lib/__tests__/yearInReviewUtils.test.ts
new file mode 100644
index 0000000..bc525b3
--- /dev/null
+++ b/src/lib/__tests__/yearInReviewUtils.test.ts
@@ -0,0 +1,125 @@
+import { describe, it, expect } from "vitest";
+import {
+    buildHourlyHeatmapFromCommitDates,
+    getMostActiveHour,
+    getMostActiveDayFromCalendar
+} from "@/lib/yearInReviewUtils";
+
+describe("buildHourlyHeatmapFromCommitDates", () => {
+    it("returns a 7x24 heatmap initialized with zeros for an empty array", () => {
+        const heatmap = buildHourlyHeatmapFromCommitDates([]);
+        expect(heatmap).toHaveLength(7);
+        heatmap.forEach(day => {
+            expect(day).toHaveLength(24);
+            expect(day.every(count => count === 0)).toBe(true);
+        });
+    });
+
+    it("correctly counts commit dates based on UTC day and hour", () => {
+        const commitDates = [
+            "2023-01-01T10:00:00Z", // Sunday (0), Hour 10
+            "2023-01-01T10:30:00Z", // Sunday (0), Hour 10
+            "2023-01-02T15:45:00Z", // Monday (1), Hour 15
+            "2023-01-07T23:59:59Z", // Saturday (6), Hour 23
+        ];
+        const heatmap = buildHourlyHeatmapFromCommitDates(commitDates);
+
+        expect(heatmap[0][10]).toBe(2);
+        expect(heatmap[1][15]).toBe(1);
+        expect(heatmap[6][23]).toBe(1);
+
+        // Verify other slots are 0
+        expect(heatmap[0][11]).toBe(0);
+        expect(heatmap[2][15]).toBe(0);
+    });
+
+    it("ignores invalid date strings", () => {
+        // Additional invalid date assertions to address regression tests mentioned
+        expect(buildHourlyHeatmapFromCommitDates(["invalid-date"])).toEqual(Array.from({ length: 7 }, () => Array.from({ length: 24 }, () => 0)));
+        const commitDates = [
+            "2023-01-01T10:00:00Z",
+            "invalid-date",
+            "not-a-date"
+        ];
+        const heatmap = buildHourlyHeatmapFromCommitDates(commitDates);
+
+        expect(heatmap[0][10]).toBe(1);
+        // All other entries should be 0
+        const totalCommits = heatmap.flat().reduce((sum, count) => sum + count, 0);
+        expect(totalCommits).toBe(1);
+    });
+});
+
+describe("getMostActiveHour", () => {
+    it("returns 0 if heatmap is malformed (not 7x24 matrix)", () => {
+        expect(getMostActiveHour([])).toBe(0);
+        expect(getMostActiveHour([[1,2,3]])).toBe(0);
+    });
+
+    it("returns 0 for malformed heatmaps with NaN or Infinity", () => {
+        const heatmapWithNaN = Array.from({ length: 7 }, () => Array.from({ length: 24 }, () => NaN));
+        expect(getMostActiveHour(heatmapWithNaN)).toBe(0);
+        const heatmapWithInfinity = Array.from({ length: 7 }, () => Array.from({ length: 24 }, () => Infinity));
+        expect(getMostActiveHour(heatmapWithInfinity)).toBe(0);
+    });
+
+    it("returns 0 for an empty heatmap (all zeros)", () => {
+        const heatmap = Array.from({ length: 7 }, () => Array.from({ length: 24 }, () => 0));
+        expect(getMostActiveHour(heatmap)).toBe(0);
+    });
+
+    it("returns the hour with the most commits across all days", () => {
+        const heatmap = Array.from({ length: 7 }, () => Array.from({ length: 24 }, () => 0));
+        heatmap[0][10] = 5; // Sunday hour 10: 5 commits
+        heatmap[1][10] = 3; // Monday hour 10: 3 commits -> Total 8
+
+        heatmap[2][15] = 4; // Tuesday hour 15: 4 commits
+        heatmap[3][15] = 5; // Wednesday hour 15: 5 commits -> Total 9
+
+        expect(getMostActiveHour(heatmap)).toBe(15);
+    });
+
+    it("returns the first encountered hour in case of a tie", () => {
+        const heatmap = Array.from({ length: 7 }, () => Array.from({ length: 24 }, () => 0));
+        heatmap[0][5] = 10; // Total 10 for hour 5
+        heatmap[0][12] = 10; // Total 10 for hour 12
+        heatmap[0][20] = 10; // Total 10 for hour 20
+
+        // Hour 5 is encountered first in the 0..23 loop
+        expect(getMostActiveHour(heatmap)).toBe(5);
+    });
+});
+
+describe("getMostActiveDayFromCalendar", () => {
+    it("returns 'Sunday' when the calendar is empty", () => {
+        expect(getMostActiveDayFromCalendar([])).toBe("Sunday");
+    });
+
+    it("correctly identifies the most active day of the week", () => {
+        const calendar = [
+            { date: "2023-01-01", count: 5 }, // Sunday
+            { date: "2023-01-02", count: 10 }, // Monday
+            { date: "2023-01-08", count: 3 },  // Sunday -> Sunday total: 8, Monday total: 10
+            { date: "2023-01-04", count: 2 },  // Wednesday -> Wednesday total: 2
+        ];
+        expect(getMostActiveDayFromCalendar(calendar)).toBe("Monday");
+    });
+
+    it("ignores days with zero or negative counts", () => {
+        const calendar = [
+            { date: "2023-01-01", count: 0 }, // Sunday
+            { date: "2023-01-02", count: -5 }, // Monday
+            { date: "2023-01-03", count: 2 }, // Tuesday
+        ];
+        expect(getMostActiveDayFromCalendar(calendar)).toBe("Tuesday");
+    });
+
+    it("returns the first encountered day in case of a tie", () => {
+        const calendar = [
+            { date: "2023-01-02", count: 10 }, // Monday (index 1)
+            { date: "2023-01-04", count: 10 }, // Wednesday (index 3)
+        ];
+        // "Monday" should be returned since it appears earlier in the [Sunday, Monday, ...] array
+        expect(getMostActiveDayFromCalendar(calendar)).toBe("Monday");
+    });
+});
diff --git a/src/lib/yearInReviewUtils.ts b/src/lib/yearInReviewUtils.ts
index 9574e09..6862c0d 100644
--- a/src/lib/yearInReviewUtils.ts
+++ b/src/lib/yearInReviewUtils.ts
@@ -13,6 +13,14 @@ export function buildHourlyHeatmapFromCommitDates(commitDates: string[]): number
 }
 
 export function getMostActiveHour(heatmap: number[][]): number {
+    const isValidHeatmap =
+        Array.isArray(heatmap) &&
+        heatmap.length === 7 &&
+        heatmap.every((row) => Array.isArray(row) && row.length === 24 && row.every((count) => Number.isFinite(count)));
+
+    if (!isValidHeatmap) {
+        return 0;
+    }
     let maxCount = -1;
     let mostActiveHour = 0;
     for (let hour = 0; hour < 24; hour += 1) {
@@ -36,7 +44,11 @@ export function getMostActiveDayFromCalendar(calendar: { date: string; count: nu
         if (day.count <= 0) {
             continue;
         }
-        const weekday = new Date(`${day.date}T00:00:00Z`).getUTCDay();
+        const parsedDate = new Date(`${day.date}T00:00:00Z`);
+        if (Number.isNaN(parsedDate.getTime())) {
+            continue;
+        }
+        const weekday = parsedDate.getUTCDay();
         totals[weekday] += day.count;
     }