LibJS: Implement Date.prototype.getTimezoneOffset

Author: https://github.com/trflynn89
Commit: https://github.com/SerenityOS/serenity/commit/6576d0291cf
Pull-request: https://github.com/SerenityOS/serenity/pull/11913
Reviewed-by: https://github.com/linusg ✅
Reviewed-by: https://github.com/nico

Userland/Libraries/LibJS/Runtime/DatePrototype.cpp

@@ -245,13 +245,15 @@ JS_DEFINE_NATIVE_FUNCTION(DatePrototype::get_time)
 // 21.4.4.11 Date.prototype.getTimezoneOffset ( ), https://tc39.es/ecma262/#sec-date.prototype.gettimezoneoffset
 JS_DEFINE_NATIVE_FUNCTION(DatePrototype::get_timezone_offset)
 {
-    auto* this_object = TRY(typed_this_object(global_object));
+    // 1. Let t be ? thisTimeValue(this value).
+    auto time = TRY(this_time_value(global_object, vm.this_value(global_object)));
 
-    if (!Value(this_object->date_value()).is_finite_number())
+    // 2. If t is NaN, return NaN.
+    if (time.is_nan())
         return js_nan();
 
-    // FIXME: Make this actually do something once we support timezones instead of just UTC
-    return Value(0);
+    // 3. Return (t - LocalTime(t)) / msPerMinute.
+    return Value((time.as_double() - local_time(time.as_double())) / Date::ms_per_minute);
 }
 
 // 21.4.4.12 Date.prototype.getUTCDate ( ), https://tc39.es/ecma262/#sec-date.prototype.getutcdate

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);
+    });
+});