LibJS: Implement Temporal.Now.plainDate()

...and ten required AOs we didn't have yet:

- BalanceISODate
- BalanceISODateTime
- BalanceISOYearMonth
- BalanceTime
- BuiltinTimeZoneGetPlainDateTimeFor
- GetISOPartsFromEpoch
- GetOffsetNanosecondsFor
- ParseTemporalTimeZone
- SystemDateTime
- ToTemporalTimeZone

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

@@ -54,4 +54,57 @@ bool is_valid_time(u8 hour, u8 minute, u8 second, u16 millisecond, u16 microseco
     return true;
 }
 
+// 4.5.6 BalanceTime ( hour, minute, second, millisecond, microsecond, nanosecond ), https://tc39.es/proposal-temporal/#sec-temporal-balancetime
+DaysAndTime balance_time(i64 hour, i64 minute, i64 second, i64 millisecond, i64 microsecond, i64 nanosecond)
+{
+    // 1. Assert: hour, minute, second, millisecond, microsecond, and nanosecond are integers.
+
+    // 2. Set microsecond to microsecond + floor(nanosecond / 1000).
+    microsecond += nanosecond / 1000;
+
+    // 3. Set nanosecond to nanosecond modulo 1000.
+    nanosecond %= 1000;
+
+    // 4. Set millisecond to millisecond + floor(microsecond / 1000).
+    millisecond += microsecond / 1000;
+
+    // 5. Set microsecond to microsecond modulo 1000.
+    microsecond %= 1000;
+
+    // 6. Set second to second + floor(millisecond / 1000).
+    second += millisecond / 1000;
+
+    // 7. Set millisecond to millisecond modulo 1000.
+    millisecond %= 1000;
+
+    // 8. Set minute to minute + floor(second / 60).
+    minute += second / 60;
+
+    // 9. Set second to second modulo 60.
+    second %= 60;
+
+    // 10. Set hour to hour + floor(minute / 60).
+    hour += minute / 60;
+
+    // 11. Set minute to minute modulo 60.
+    minute %= 60;
+
+    // 12. Let days be floor(hour / 24).
+    u8 days = hour / 24;
+
+    // 13. Set hour to hour modulo 24.
+    hour %= 24;
+
+    // 14. Return the new Record { [[Days]]: days, [[Hour]]: hour, [[Minute]]: minute, [[Second]]: second, [[Millisecond]]: millisecond, [[Microsecond]]: microsecond, [[Nanosecond]]: nanosecond }.
+    return DaysAndTime {
+        .days = static_cast<i32>(days),
+        .hour = static_cast<u8>(hour),
+        .minute = static_cast<u8>(minute),
+        .second = static_cast<u8>(second),
+        .millisecond = static_cast<u16>(millisecond),
+        .microsecond = static_cast<u16>(microsecond),
+        .nanosecond = static_cast<u16>(nanosecond),
+    };
+}
+
 }