LibJS: Implement TODO()'d parts of balance_duration()

Massive :yakstack:, so I understand why it was skipped in the first
place :^)

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

@@ -10,6 +10,7 @@
 #include <LibJS/Runtime/Object.h>
 #include <LibJS/Runtime/Temporal/AbstractOperations.h>
 #include <LibJS/Runtime/Temporal/Calendar.h>
+#include <LibJS/Runtime/Temporal/Duration.h>
 #include <LibJS/Runtime/Temporal/Instant.h>
 #include <LibJS/Runtime/Temporal/PlainDateTime.h>
 #include <LibJS/Runtime/Temporal/PlainTime.h>
@@ -38,6 +39,39 @@ void PlainTime::visit_edges(Visitor& visitor)
     visitor.visit(&m_calendar);
 }
 
+// 4.5.1 DifferenceTime ( h1, min1, s1, ms1, mus1, ns1, h2, min2, s2, ms2, mus2, ns2 ), https://tc39.es/proposal-temporal/#sec-temporal-differencetime
+BalancedDuration difference_time(u8 hour1, u8 minute1, u8 second1, u16 millisecond1, u16 microsecond1, u16 nanosecond1, u8 hour2, u8 minute2, u8 second2, u16 millisecond2, u16 microsecond2, u16 nanosecond2)
+{
+    // Assert: h1, min1, s1, ms1, mus1, ns1, h2, min2, s2, ms2, mus2, and ns2 are integers.
+
+    // 2. Let hours be h2 − h1.
+    auto hours = hour2 - hour1;
+
+    // 3. Let minutes be min2 − min1.
+    auto minutes = minute2 - minute1;
+
+    // 4. Let seconds be s2 − s1.
+    auto seconds = second2 - second1;
+
+    // 5. Let milliseconds be ms2 − ms1.
+    auto milliseconds = millisecond2 - millisecond1;
+
+    // 6. Let microseconds be mus2 − mus1.
+    auto microseconds = microsecond2 - microsecond1;
+
+    // 7. Let nanoseconds be ns2 − ns1.
+    auto nanoseconds = nanosecond2 - nanosecond1;
+
+    // 8. Let sign be ! DurationSign(0, 0, 0, 0, hours, minutes, seconds, milliseconds, microseconds, nanoseconds).
+    auto sign = duration_sign(0, 0, 0, 0, hours, minutes, seconds, milliseconds, microseconds, nanoseconds);
+
+    // 9. Let bt be ! BalanceTime(hours × sign, minutes × sign, seconds × sign, milliseconds × sign, microseconds × sign, nanoseconds × sign).
+    auto bt = balance_time(hours * sign, minutes * sign, seconds * sign, milliseconds * sign, microseconds * sign, nanoseconds * sign);
+
+    // 10. Return the Record { [[Days]]: bt.[[Days]] × sign, [[Hours]]: bt.[[Hour]] × sign, [[Minutes]]: bt.[[Minute]] × sign, [[Seconds]]: bt.[[Second]] × sign, [[Milliseconds]]: bt.[[Millisecond]] × sign, [[Microseconds]]: bt.[[Microsecond]] × sign, [[Nanoseconds]]: bt.[[Nanosecond]] × sign }.
+    return BalancedDuration { .days = static_cast<double>(bt.days * sign), .hours = static_cast<double>(bt.hour * sign), .minutes = static_cast<double>(bt.minute * sign), .seconds = static_cast<double>(bt.second * sign), .milliseconds = static_cast<double>(bt.millisecond * sign), .microseconds = static_cast<double>(bt.microsecond * sign), .nanoseconds = static_cast<double>(bt.nanosecond * sign) };
+}
+
 // 4.5.2 ToTemporalTime ( item [ , overflow ] ), https://tc39.es/proposal-temporal/#sec-temporal-totemporaltime
 ThrowCompletionOr<PlainTime*> to_temporal_time(GlobalObject& global_object, Value item, Optional<StringView> overflow)
 {