LibJS: Fix calculation overflow in parse_temporal_time_zone_string()

As all variables and numeric literals in the expression have an integral
data type, it would evaluate to an int and could easily overflow as
we're multiplying seconds with 10^9.

Introduce a floating point literal into the expression to make it result
in a double.

Userland/Libraries/LibJS/Runtime/Temporal/AbstractOperations.cpp

@@ -1670,8 +1670,11 @@ ThrowCompletionOr<TemporalTimeZone> parse_temporal_time_zone_string(GlobalObject
             // i. Let nanoseconds be 0.
             nanoseconds = 0;
         }
+
         // i. Let offsetNanoseconds be sign × (((hours × 60 + minutes) × 60 + seconds) × 10^9 + nanoseconds).
-        auto offset_nanoseconds = sign * (((hours * 60 + minutes) * 60 + seconds) * 1000000000 + nanoseconds);
+        // NOTE: Decimal point in 10^9 is important, otherwise it's all integers and the result overflows!
+        auto offset_nanoseconds = sign * (((hours * 60 + minutes) * 60 + seconds) * 1000000000.0 + nanoseconds);
+
         // j. Let offsetString be ! FormatTimeZoneOffsetString(offsetNanoseconds).
         offset = format_time_zone_offset_string(offset_nanoseconds);
     }

Userland/Libraries/LibJS/Tests/builtins/Temporal/TimeZone/TimeZone.from.js

@@ -4,20 +4,43 @@ describe("normal behavior", () => {
     });
 
     test("basic functionality", () => {
+        // From object
         const timeZone = new Temporal.TimeZone("UTC");
         const timeZoneLike = {};
         const zonedDateTimeLike = { timeZone: {} };
         expect(Temporal.TimeZone.from(timeZone)).toBe(timeZone);
         expect(Temporal.TimeZone.from(timeZoneLike)).toBe(timeZoneLike);
         expect(Temporal.TimeZone.from(zonedDateTimeLike)).toBe(zonedDateTimeLike.timeZone);
-        expect(Temporal.TimeZone.from("UTC").id).toBe("UTC");
-        expect(Temporal.TimeZone.from("GMT").id).toBe("UTC");
-        expect(Temporal.TimeZone.from("Etc/UTC").id).toBe("UTC");
-        expect(Temporal.TimeZone.from("Etc/GMT").id).toBe("UTC");
-        // FIXME: https://github.com/tc39/proposal-temporal/issues/1993
-        // expect(Temporal.TimeZone.from("Etc/GMT+12").id).toBe("Etc/GMT+12");
-        // expect(Temporal.TimeZone.from("Etc/GMT-12").id).toBe("Etc/GMT-12");
-        expect(Temporal.TimeZone.from("Europe/London").id).toBe("Europe/London");
-        expect(Temporal.TimeZone.from("Europe/Isle_of_Man").id).toBe("Europe/London");
+
+        // From string
+        const values = [
+            ["UTC", "UTC"],
+            ["GMT", "UTC"],
+            ["Etc/UTC", "UTC"],
+            ["Etc/GMT", "UTC"],
+            // FIXME: https://github.com/tc39/proposal-temporal/issues/1993
+            // ["Etc/GMT+12", "Etc/GMT+12"],
+            // ["Etc/GMT-12", "Etc/GMT-12"],
+            ["Europe/London", "Europe/London"],
+            ["Europe/Isle_of_Man", "Europe/London"],
+            ["1970-01-01+01", "+01:00"],
+            ["1970-01-01+01[-12:34]", "+01:00"],
+            ["1970-01-01T00:00:00+01", "+01:00"],
+            ["1970-01-01T00:00:00.000000000+01", "+01:00"],
+            ["1970-01-01T00:00:00.000000000+01:00:00", "+01:00"],
+            ["1970-01-01+12:34", "+12:34"],
+            ["1970-01-01+12:34:56", "+12:34:56"],
+            // FIXME: These currently crash :^(
+            // ["1970-01-01+12:34:56.789", "+12:34:56.789"],
+            // ["1970-01-01+12:34:56.789[-01:00]", "+12:34:56.789"],
+            ["1970-01-01-12:34", "-12:34"],
+            ["1970-01-01-12:34:56", "-12:34:56"],
+            // FIXME: These currently crash :^(
+            // ["1970-01-01-12:34:56.789", "-12:34:56.789"],
+            // ["1970-01-01-12:34:56.789[+01:00]", "-12:34:56.789"],
+        ];
+        for (const [arg, expected] of values) {
+            expect(Temporal.TimeZone.from(arg).id).toBe(expected);
+        }
     });
 });