LibJS: Implement Date.prototype.getTimezoneOffset

Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.getTimezoneOffset.js

@@ -0,0 +1,30 @@
+describe("errors", () => {
+    test("called on non-Date object", () => {
+        expect(() => {
+            Date.prototype.getTimezoneOffset();
+        }).toThrowWithMessage(TypeError, "Not an object of type Date");
+    });
+});
+
+describe("correct behavior", () => {
+    test("NaN", () => {
+        const d = new Date(NaN);
+        expect(d.getTimezoneOffset()).toBeNaN();
+    });
+
+    test("time clip", () => {
+        const d = new Date(-8.65e15);
+        expect(d.getTimezoneOffset()).toBeNaN();
+    });
+
+    test("basic functionality", () => {
+        // Exact return values from getTimezoneOffset depend on the time zone of the host machine.
+        // So we can't test exact values, but that value should not change here.
+        const d0 = new Date(Date.parse("1989-01-23T14:30-00:00"));
+        const d1 = new Date(Date.parse("1989-01-23T14:30-05:00"));
+
+        const offset0 = d0.getTimezoneOffset();
+        const offset1 = d1.getTimezoneOffset();
+        expect(offset0).toBe(offset1);
+    });
+});