Split out SubtractTime as an abstract operation

It's odd that we have AddTime but not SubtractTime. Regardless, we'll
need to have it as a separate operation since it will be used from two
places for negative duration support.

spec/time.html

@@ -212,7 +212,6 @@ <h1>Temporal.Time.prototype.plus ( _temporalDurationLike_ [ , _options_ ] )</h1>
         1. Let _duration_ be ? ToLimitedTemporalDuration(_temporalDurationLike_, « »).
         1. Perform ? ToTemporalDisambiguation(_options_).
         1. Let _result_ be ? AddTime(_temporalTime_.[[Hour]], _temporalTime_.[[Minute]], _temporalTime_.[[Second]], _temporalTime_.[[Millisecond]], _temporalTime_.[[Microsecond]], _temporalTime_.[[Nanosecond]], _duration_.[[Hours]], _duration_.[[Minutes]], _duration_.[[Seconds]], _duration_.[[Milliseconds]], _duration_.[[Microseconds]], _duration_.[[Nanoseconds]]).
-        1. Let _result_ be ? BalanceTime(_hour_, _minute_, _second_, _millisecond_, _microsecond_, _nanosecond_).
         1. Return ? CreateTemporalTimeFromInstance(_temporalTime_, _result_.[[Hour]], _result_.[[Minute]], _result_.[[Second]], _result_.[[Millisecond]], _result_.[[Microsecond]], _result_.[[Nanosecond]]).
       </emu-alg>
     </emu-clause>
@@ -228,13 +227,7 @@ <h1>Temporal.Time.prototype.minus ( _temporalDurationLike_ [ , _options_ ] )</h1
         1. Perform ? RequireInternalSlot(_temporalTime_, [[InitializedTemporalTime]]).
         1. Let _duration_ be ? ToLimitedTemporalDuration(_temporalDurationLike_, « »).
         1. Perform ? ToTemporalDisambiguation(_options_).
-        1. Let _hour_ be _temporalTime_.[[Hour]] - _duration_.[[Hour]].
-        1. Let _minute_ be _temporalTime_.[[Minute]] - _duration_.[[Minute]].
-        1. Let _second_ be _temporalTime_.[[Second]] - _duration_.[[Second]].
-        1. Let _millisecond_ be _temporalTime_.[[Millisecond]] - _duration_.[[Millisecond]].
-        1. Let _microsecond_ be _temporalTime_.[[Microsecond]] - _duration_.[[Microsecond]].
-        1. Let _nanosecond_ be _temporalTime_.[[Nanosecond]] - _duration_.[[Nanosecond]].
-        1. Let _result_ be ? BalanceTime(_hour_, _minute_, _second_, _millisecond_, _microsecond_, _nanosecond_).
+        1. Let _result_ be ? SubtractTime(_temporalTime_.[[Hour]], _temporalTime_.[[Minute]], _temporalTime_.[[Second]], _temporalTime_.[[Millisecond]], _temporalTime_.[[Microsecond]], _temporalTime_.[[Nanosecond]], _duration_.[[Hours]], _duration_.[[Minutes]], _duration_.[[Seconds]], _duration_.[[Milliseconds]], _duration_.[[Microseconds]], _duration_.[[Nanoseconds]]).
         1. Return ? CreateTemporalTimeFromInstance(_temporalTime_, _result_.[[Hour]], _result_.[[Minute]], _result_.[[Second]], _result_.[[Millisecond]], _result_.[[Microsecond]], _result_.[[Nanosecond]]).
       </emu-alg>
     </emu-clause>
@@ -717,5 +710,19 @@ <h1>AddTime ( _hour_, _minute_, _second_, _millisecond_, _microsecond_, _nanosec
         1. Return ? BalanceTime(_hour_, _minute_, _second_, _millisecond_, _microsecond_, _nanosecond_).
       </emu-alg>
     </emu-clause>
+
+    <emu-clause id="sec-temporal-subtracttime" aoid="SubtractTime">
+      <h1>SubtractTime ( _hour_, _minute_, _second_, _millisecond_, _microsecond_, _nanosecond_, _hours_, _minutes_, _seconds_, _milliseconds_, _microseconds_, _nanoseconds_ )</h1>
+      <emu-alg>
+        1. Assert: _hour_, _minute_, _second_, _millisecond_, _microsecond_, _nanosecond_, _hours_, _minutes_, _seconds_, _milliseconds_, _microseconds_, and _nanoseconds_ are integer Number values.
+        1. Let _hour_ be _temporalTime_.[[Hour]] - _duration_.[[Hour]].
+        1. Let _minute_ be _temporalTime_.[[Minute]] - _duration_.[[Minute]].
+        1. Let _second_ be _temporalTime_.[[Second]] - _duration_.[[Second]].
+        1. Let _millisecond_ be _temporalTime_.[[Millisecond]] - _duration_.[[Millisecond]].
+        1. Let _microsecond_ be _temporalTime_.[[Microsecond]] - _duration_.[[Microsecond]].
+        1. Let _nanosecond_ be _temporalTime_.[[Nanosecond]] - _duration_.[[Nanosecond]].
+        1. Return ? BalanceTime(_hour_, _minute_, _second_, _millisecond_, _microsecond_, _nanosecond_).
+      </emu-alg>
+    </emu-clause>
   </emu-clause>
 </emu-clause>