diff --git a/lib/bills/urgency.test.ts b/lib/bills/urgency.test.ts
new file mode 100644
index 0000000..72b5012
--- /dev/null
+++ b/lib/bills/urgency.test.ts
@@ -0,0 +1,105 @@
+import { describe, it, expect, beforeEach, afterEach, vi } from "vitest";
+import { daysDiff, computeUrgency, computeDaysInfo } from "./urgency";
+
+describe("Bills Urgency", () => {
+  beforeEach(() => {
+    vi.useFakeTimers();
+  });
+
+  afterEach(() => {
+    vi.useRealTimers();
+  });
+
+  describe("daysDiff", () => {
+    it("returns 0 for today", () => {
+      vi.setSystemTime(new Date(2024, 0, 15, 12, 0, 0));
+      expect(daysDiff("2024-01-15T00:00:00")).toBe(0);
+      expect(daysDiff("2024-01-15T23:59:59")).toBe(0);
+    });
+
+    it("returns negative for past dates (overdue)", () => {
+      vi.setSystemTime(new Date(2024, 0, 15, 12, 0, 0));
+      expect(daysDiff("2024-01-14T10:00:00")).toBe(-1);
+      expect(daysDiff("2024-01-01T00:00:00")).toBe(-14);
+    });
+
+    it("returns positive for future dates (upcoming)", () => {
+      vi.setSystemTime(new Date(2024, 0, 15, 12, 0, 0));
+      expect(daysDiff("2024-01-16T10:00:00")).toBe(1);
+      expect(daysDiff("2024-01-20T00:00:00")).toBe(5);
+    });
+
+    it("handles end-of-month rollover", () => {
+      vi.setSystemTime(new Date(2024, 0, 31, 12, 0, 0));
+      expect(daysDiff("2024-02-01T00:00:00")).toBe(1);
+    });
+
+    it("handles leap year boundaries", () => {
+      vi.setSystemTime(new Date(2024, 1, 28, 12, 0, 0));
+      expect(daysDiff("2024-02-29T00:00:00")).toBe(1);
+      expect(daysDiff("2024-03-01T00:00:00")).toBe(2);
+
+      // non-leap year
+      vi.setSystemTime(new Date(2023, 1, 28, 12, 0, 0));
+      expect(daysDiff("2023-03-01T00:00:00")).toBe(1);
+    });
+
+    it("ignores timezone offset differences by standardizing calendar days", () => {
+      vi.setSystemTime(new Date(2024, 0, 15, 23, 0, 0));
+      expect(daysDiff("2024-01-15T01:00:00")).toBe(0);
+      expect(daysDiff("2024-01-16T01:00:00")).toBe(1);
+    });
+  });
+
+  describe("computeUrgency", () => {
+    beforeEach(() => {
+      vi.setSystemTime(new Date(2024, 4, 15, 12, 0, 0));
+    });
+
+    it("returns overdue for past dates", () => {
+      expect(computeUrgency("2024-05-14T00:00:00")).toBe("overdue");
+      expect(computeUrgency("2024-05-01T00:00:00")).toBe("overdue");
+    });
+
+    it("returns urgent for today and next 3 days", () => {
+      expect(computeUrgency("2024-05-15T00:00:00")).toBe("urgent"); // 0
+      expect(computeUrgency("2024-05-16T00:00:00")).toBe("urgent"); // 1
+      expect(computeUrgency("2024-05-17T00:00:00")).toBe("urgent"); // 2
+      expect(computeUrgency("2024-05-18T00:00:00")).toBe("urgent"); // 3
+    });
+
+    it("returns upcoming for 4 days and beyond", () => {
+      expect(computeUrgency("2024-05-19T00:00:00")).toBe("upcoming"); // 4
+      expect(computeUrgency("2024-06-01T00:00:00")).toBe("upcoming"); // far future
+    });
+
+    it("asserts the urgent-upcoming boundary", () => {
+      expect(computeUrgency("2024-05-18T00:00:00")).toBe("urgent");
+      expect(computeUrgency("2024-05-19T00:00:00")).toBe("upcoming");
+    });
+  });
+
+  describe("computeDaysInfo", () => {
+    beforeEach(() => {
+      vi.setSystemTime(new Date(2024, 4, 15, 12, 0, 0));
+    });
+
+    it("formats overdue dates correctly", () => {
+      expect(computeDaysInfo("2024-05-14T00:00:00")).toBe("1d overdue");
+      expect(computeDaysInfo("2024-05-05T00:00:00")).toBe("10d overdue");
+    });
+
+    it("formats due today correctly", () => {
+      expect(computeDaysInfo("2024-05-15T00:00:00")).toBe("Due today");
+    });
+
+    it("formats due tomorrow correctly", () => {
+      expect(computeDaysInfo("2024-05-16T00:00:00")).toBe("Due tomorrow");
+    });
+
+    it("formats upcoming dates correctly", () => {
+      expect(computeDaysInfo("2024-05-17T00:00:00")).toBe("2d left");
+      expect(computeDaysInfo("2024-05-25T00:00:00")).toBe("10d left");
+    });
+  });
+});
diff --git a/lib/bills/urgency.ts b/lib/bills/urgency.ts
index 50a4369..63ab6ca 100644
--- a/lib/bills/urgency.ts
+++ b/lib/bills/urgency.ts
@@ -3,7 +3,7 @@
  * Keeps mock data and API responses in sync with the same logic.
  */
 
-export type BillUrgency = 'overdue' | 'urgent' | 'upcoming' | 'paid';
+export type BillUrgency = "overdue" | "urgent" | "upcoming" | "paid";
 
 /** Returns days difference: negative = overdue, 0 = today, positive = future */
 export function daysDiff(dueDateStr: string): number {
@@ -17,15 +17,15 @@ export function daysDiff(dueDateStr: string): number {
 
 export function computeUrgency(dueDateStr: string): BillUrgency {
   const diff = daysDiff(dueDateStr);
-  if (diff < 0) return 'overdue';
-  if (diff <= 3) return 'urgent';
-  return 'upcoming';
+  if (diff < 0) return "overdue";
+  if (diff <= 3) return "urgent";
+  return "upcoming";
 }
 
 export function computeDaysInfo(dueDateStr: string): string {
   const diff = daysDiff(dueDateStr);
   if (diff < 0) return `${Math.abs(diff)}d overdue`;
-  if (diff === 0) return 'Due today';
-  if (diff === 1) return 'Due tomorrow';
+  if (diff === 0) return "Due today";
+  if (diff === 1) return "Due tomorrow";
   return `${diff}d left`;
 }
diff --git a/vitest.config.mts b/vitest.config.mts
index 6b5c80d..75c0611 100644
--- a/vitest.config.mts
+++ b/vitest.config.mts
@@ -5,6 +5,7 @@ export default defineConfig({
   test: {
     include: [
       'lib/contracts/**/*.test.ts',
+      'lib/**/*.test.ts',
       'tests/unit/**/*.test.ts',
       'tests/unit/**/*.test.cjs',
       'tests/integration/**/*.test.ts',
@@ -22,6 +23,7 @@ export default defineConfig({
       include: ['lib/contracts/**/*.ts', 'app/**/*.ts', 'lib/**/*.ts'],
       exclude: [
         'lib/contracts/**/*.test.ts',
+        'lib/**/*.test.ts',
         'tests/**',
       ],
     },