diff --git a/test/defu.test.ts b/test/defu.test.ts
index 7f12aa2..19d490d 100644
--- a/test/defu.test.ts
+++ b/test/defu.test.ts
@@ -50,7 +50,7 @@ describe("defu", () => {
     }>();
   });
 
-  it.skip("should avoid merging objects with custom constructor", () => {
+  it("should avoid merging objects with custom constructor", () => {
     class Test {
       // eslint-disable-next-line no-useless-constructor
       constructor(public value: string) {}
@@ -59,11 +59,11 @@ describe("defu", () => {
     expect(result).toEqual({ test: new Test("a") });
   });
 
-  it.skip("should assign date properly", () => {
+  it("should assign date properly", () => {
     const date1 = new Date("2020-01-01");
     const date2 = new Date("2020-01-02");
     const result = defu({ date: date1 }, { date: date2 });
-    expect(result).toEqual({ date: date2 });
+    expect(result).toEqual({ date: date1 });
   });
 
   it("should correctly merge different object types", () => {
diff --git a/test/utils.test.ts b/test/utils.test.ts
new file mode 100644
index 0000000..3dcef5b
--- /dev/null
+++ b/test/utils.test.ts
@@ -0,0 +1,37 @@
+import { it, expect } from "vitest";
+import { isPlainObject } from "../src/_utils";
+
+it("isPlainObject", () => {
+  expect(isPlainObject(undefined)).toBe(false);
+  expect(isPlainObject(0)).toBe(false);
+  expect(isPlainObject(0n)).toBe(false);
+  expect(isPlainObject("")).toBe(false);
+  expect(isPlainObject(true)).toBe(false);
+  expect(isPlainObject(Symbol(""))).toBe(false);
+  expect(isPlainObject(() => {})).toBe(false);
+  expect(isPlainObject(function namedFunc() {})).toBe(false);
+  expect(isPlainObject(null)).toBe(false);
+  expect(isPlainObject({})).toBe(true);
+  expect(isPlainObject(Math)).toBe(false);
+  expect(isPlainObject(new Set([]))).toBe(false);
+  expect(isPlainObject(new ArrayBuffer(0))).toBe(false);
+  expect(isPlainObject(Promise.resolve())).toBe(false);
+  expect(isPlainObject(Object.create(null))).toBe(true);
+  expect(isPlainObject(new Intl.Locale("en"))).toBe(false);
+  // eslint-disable-next-line no-new-object
+  expect(isPlainObject(new Object({ prop: true }))).toBe(true);
+  expect(isPlainObject(new (class Class {})())).toBe(false);
+  expect(isPlainObject([])).toBe(false);
+  expect(isPlainObject(/regexp/)).toBe(false);
+  expect(isPlainObject(new Error("test"))).toBe(false);
+  expect(isPlainObject(new Date())).toBe(false);
+  expect(
+    isPlainObject(
+      (function () {
+        // eslint-disable-next-line prefer-rest-params
+        return arguments;
+      })(),
+    ),
+  ).toBe(false);
+  // expect(isPlainObject(new Proxy({}, {}))).toBe(false); // TODO
+});