LibJS: Implement Date.UTC according to the spec

This fixes all failing Date.UTC test262 tests, which failed due to not
handling invalid input and evaluating inputs out of order. But this also
avoids using timegm(), which doesn't work on macOS for years before 1900
(they simply return -1 for those years).

Partially addresses #4651. Date.parse.js still fails.

Userland/Libraries/LibJS/Runtime/DateConstructor.cpp

@@ -302,24 +302,40 @@ JS_DEFINE_NATIVE_FUNCTION(DateConstructor::parse)
 // 21.4.3.4 Date.UTC ( year [ , month [ , date [ , hours [ , minutes [ , seconds [ , ms ] ] ] ] ] ] ), https://tc39.es/ecma262/#sec-date.utc
 JS_DEFINE_NATIVE_FUNCTION(DateConstructor::utc)
 {
-    auto arg_or = [&vm, &global_object](size_t i, i32 fallback) -> ThrowCompletionOr<i32> {
-        return vm.argument_count() > i ? vm.argument(i).to_i32(global_object) : fallback;
+    auto arg_or = [&vm, &global_object](size_t i, i32 fallback) -> ThrowCompletionOr<Value> {
+        return vm.argument_count() > i ? vm.argument(i).to_number(global_object) : Value(fallback);
     };
-    int year = TRY(vm.argument(0).to_i32(global_object));
-    if (year >= 0 && year <= 99)
-        year += 1900;
 
-    struct tm tm = {};
-    tm.tm_year = year - 1900;
-    tm.tm_mon = TRY(arg_or(1, 0)); // 0-based in both tm and JavaScript
-    tm.tm_mday = TRY(arg_or(2, 1));
-    tm.tm_hour = TRY(arg_or(3, 0));
-    tm.tm_min = TRY(arg_or(4, 0));
-    tm.tm_sec = TRY(arg_or(5, 0));
-    // timegm() doesn't read tm.tm_wday and tm.tm_yday, no need to fill them in.
+    // 1. Let y be ? ToNumber(year).
+    auto year = TRY(vm.argument(0).to_number(global_object));
+    // 2. If month is present, let m be ? ToNumber(month); else let m be +0𝔽.
+    auto month = TRY(arg_or(1, 0));
+    // 3. If date is present, let dt be ? ToNumber(date); else let dt be 1𝔽.
+    auto date = TRY(arg_or(2, 1));
+    // 4. If hours is present, let h be ? ToNumber(hours); else let h be +0𝔽.
+    auto hours = TRY(arg_or(3, 0));
+    // 5. If minutes is present, let min be ? ToNumber(minutes); else let min be +0𝔽.
+    auto minutes = TRY(arg_or(4, 0));
+    // 6. If seconds is present, let s be ? ToNumber(seconds); else let s be +0𝔽.
+    auto seconds = TRY(arg_or(5, 0));
+    // 7. If ms is present, let milli be ? ToNumber(ms); else let milli be +0𝔽.
+    auto milliseconds = TRY(arg_or(6, 0));
 
-    int milliseconds = TRY(arg_or(6, 0));
-    return Value(1000.0 * timegm(&tm) + milliseconds);
+    // 8. If y is NaN, let yr be NaN.
+    // 9. Else,
+    if (!year.is_nan()) {
+        // a. Let yi be ! ToIntegerOrInfinity(y).
+        auto year_double = MUST(year.to_integer_or_infinity(global_object));
+
+        // b. If 0 ≤ yi ≤ 99, let yr be 1900𝔽 + 𝔽(yi); otherwise, let yr be y.
+        if (0 <= year_double && year_double <= 99)
+            year = Value(1900 + year_double);
+    }
+
+    // 10. Return TimeClip(MakeDate(MakeDay(yr, m, dt), MakeTime(h, min, s, milli))).
+    auto day = make_day(global_object, year, month, date);
+    auto time = make_time(global_object, hours, minutes, seconds, milliseconds);
+    return time_clip(global_object, make_date(day, time));
 }
 
 }

Userland/Libraries/LibJS/Tests/builtins/Date/Date.UTC.js

@@ -53,3 +53,20 @@ test("out of range", () => {
     expect(Date.UTC(2020, 1, 15, 12, 30, 30, -2345)).toBe(1581769827655);
     expect(Date.UTC(2020, 1, 15, 12, 30, 30, 2345)).toBe(1581769832345);
 });
+
+test("special values", () => {
+    [Infinity, -Infinity, NaN].forEach(value => {
+        expect(Date.UTC(value)).toBeNaN();
+        expect(Date.UTC(0, value)).toBeNaN();
+        expect(Date.UTC(0, 0, value)).toBeNaN();
+        expect(Date.UTC(0, 0, 1, value)).toBeNaN();
+        expect(Date.UTC(0, 0, 1, 0, value)).toBeNaN();
+        expect(Date.UTC(0, 0, 1, 0, 0, value)).toBeNaN();
+        expect(Date.UTC(0, 0, 1, 0, 0, 0, value)).toBeNaN();
+    });
+});
+
+test("time clip", () => {
+    expect(Date.UTC(275760, 8, 13, 0, 0, 0, 0)).toBe(8.64e15);
+    expect(Date.UTC(275760, 8, 13, 0, 0, 0, 1)).toBeNaN();
+});