diff --git a/tests/funding-deadline-validator.test.ts b/tests/funding-deadline-validator.test.ts
index 6ddf482..b4c8afc 100644
--- a/tests/funding-deadline-validator.test.ts
+++ b/tests/funding-deadline-validator.test.ts
@@ -114,6 +114,52 @@ describe("validateFundingDeadline", () => {
     expect(validateFundingDeadline(iso, NOW)).toBeNull();
   });
 
+  describe("timezone normalization", () => {
+    const originalTz = process.env.TZ;
+
+    beforeEach(() => {
+      process.env.TZ = "America/New_York";
+    });
+
+    afterEach(() => {
+      process.env.TZ = originalTz;
+    });
+
+    it("evaluates equivalent instants identically regardless of string format", () => {
+      const utcString = "2026-03-03T12:00:00.000Z";
+      const offsetString = "2026-03-03T14:00:00.000+02:00";
+      expect(validateFundingDeadline(utcString, NOW)).toBeNull();
+      expect(validateFundingDeadline(offsetString, NOW)).toBeNull();
+    });
+
+    it("rejects equivalent past deadlines across timezones", () => {
+      const utcString = "2026-03-01T11:00:00.000Z";
+      const offsetString = "2026-03-01T06:00:00.000-05:00";
+      expect(validateFundingDeadline(utcString, NOW)).toMatchObject({
+        code: "DUE_DATE_IN_PAST",
+      });
+      expect(validateFundingDeadline(offsetString, NOW)).toMatchObject({
+        code: "DUE_DATE_IN_PAST",
+      });
+    });
+
+    it("enforces the exact boundary consistently across timezones", () => {
+      const utcBoundary = "2026-03-02T12:00:00.000Z";
+      const offsetBoundary = "2026-03-02T17:30:00.000+05:30";
+      expect(validateFundingDeadline(utcBoundary, NOW)).toBeNull();
+      expect(validateFundingDeadline(offsetBoundary, NOW)).toBeNull();
+
+      const utcTooSoon = "2026-03-02T11:59:59.999Z";
+      const offsetTooSoon = "2026-03-02T17:29:59.999+05:30";
+      expect(validateFundingDeadline(utcTooSoon, NOW)).toMatchObject({
+        code: "DUE_DATE_TOO_SOON",
+      });
+      expect(validateFundingDeadline(offsetTooSoon, NOW)).toMatchObject({
+        code: "DUE_DATE_TOO_SOON",
+      });
+    });
+  });
+
   describe("clock source", () => {
     afterEach(() => {
       jest.useRealTimers();
diff --git a/tests/investor-return.test.ts b/tests/investor-return.test.ts
index b8f30ca..b0c5fa9 100644
--- a/tests/investor-return.test.ts
+++ b/tests/investor-return.test.ts
@@ -5,11 +5,26 @@ describe("computeInvestorReturn", () => {
     expect(computeInvestorReturn(2000n, 4000n, 4400n)).toBe(2200n);
   });
 
-  it("floors the result when division is not exact, without rounding up", () => {
+  it("floors the result when division is not exact (repeating fractional ratios), without rounding up", () => {
     // (1000 * 1000) / 3000 = 333.33... -> floors to 333
     expect(computeInvestorReturn(1000n, 3000n, 1000n)).toBe(333n);
   });
 
+  it("handles repeating fractional ratios correctly in edge cases", () => {
+    // e.g., 2/3 of 1000 = 666
+    expect(computeInvestorReturn(2000n, 3000n, 1000n)).toBe(666n);
+  });
+
+  it("returns 0 when yield (settledProceeds) is zero", () => {
+    expect(computeInvestorReturn(1000n, 5000n, 0n)).toBe(0n);
+  });
+
+  it("handles maximum supported yield without overflow and exact division", () => {
+    // 64-bit signed integer max
+    const int64Max = 9223372036854775807n;
+    expect(computeInvestorReturn(1000n, 2000n, int64Max)).toBe(int64Max / 2n);
+  });
+
   it("returns the full settled proceeds when investedAmount equals totalFunded", () => {
     expect(computeInvestorReturn(5000n, 5000n, 7500n)).toBe(7500n);
   });
@@ -27,4 +42,9 @@ describe("computeInvestorReturn", () => {
     expect(() => computeInvestorReturn(-1n, 1000n, 1000n)).toThrow(RangeError);
     expect(() => computeInvestorReturn(1n, 1000n, -1000n)).toThrow(RangeError);
   });
+
+  it("asserts integer output at the service boundary (bigint return type)", () => {
+    const result = computeInvestorReturn(1000n, 3000n, 1000n);
+    expect(typeof result).toBe("bigint");
+  });
 });