Introduce abstract operation TotalDurationNanoseconds

This operation gives the total number of nanoseconds in an exact
duration as a bigint, not losing precision. This will be reused in
Duration.compare().

This operation takes an "offset shift" parameter, used if there are days
that need to be balanced down to hours. If it's 0 (as is the case
everywhere in this commit) then days are always 24 hours. We will start
using nonzero values for this in Duration.compare().

See: #856

polyfill/lib/ecmascript.mjs

@@ -1481,7 +1481,7 @@ export const ES = ObjectAssign({}, ES2020, {
     if (minutes) timeParts.push(`${formatNumber(Math.abs(minutes))}M`);
 
     const secondParts = [];
-    let total = bigInt(seconds).times(1000).plus(ms).times(1000).plus(µs).times(1000).plus(ns);
+    let total = ES.TotalDurationNanoseconds(0, 0, 0, seconds, ms, µs, ns, 0);
     ({ quotient: total, remainder: ns } = total.divmod(1000));
     ({ quotient: total, remainder: µs } = total.divmod(1000));
     ({ quotient: seconds, remainder: ms } = total.divmod(1000));
@@ -1872,13 +1872,26 @@ export const ES = ObjectAssign({}, ES2020, {
     }
     return { year, month, years, months };
   },
-  BalanceDuration: (days, hours, minutes, seconds, milliseconds, microseconds, nanoseconds, largestUnit) => {
+  TotalDurationNanoseconds: (days, hours, minutes, seconds, milliseconds, microseconds, nanoseconds, offsetShift) => {
+    if (days !== 0) nanoseconds = bigInt(nanoseconds).subtract(offsetShift);
     hours = bigInt(hours).add(bigInt(days).multiply(24));
     minutes = bigInt(minutes).add(hours.multiply(60));
     seconds = bigInt(seconds).add(minutes.multiply(60));
     milliseconds = bigInt(milliseconds).add(seconds.multiply(1000));
     microseconds = bigInt(microseconds).add(milliseconds.multiply(1000));
-    nanoseconds = bigInt(nanoseconds).add(microseconds.multiply(1000));
+    return bigInt(nanoseconds).add(microseconds.multiply(1000));
+  },
+  BalanceDuration: (days, hours, minutes, seconds, milliseconds, microseconds, nanoseconds, largestUnit) => {
+    nanoseconds = ES.TotalDurationNanoseconds(
+      days,
+      hours,
+      minutes,
+      seconds,
+      milliseconds,
+      microseconds,
+      nanoseconds,
+      0
+    );
     const sign = nanoseconds.lesser(0) ? -1 : 1;
     nanoseconds = nanoseconds.abs();
     microseconds = milliseconds = seconds = minutes = hours = days = bigInt.zero;

spec/duration.html

@@ -817,15 +817,27 @@ <h1>CreateTemporalDurationFromStatic ( _constructor_, _years_, _months_, _weeks_
       </emu-alg>
     </emu-clause>
 
-    <emu-clause id="sec-temporal-balanceduration" aoid="BalanceDuration">
-      <h1>BalanceDuration ( _days_, _hours_, _minutes_, _seconds_, _milliseconds_, _microseconds_, _nanoseconds_, _largestUnit_ )</h1>
+    <emu-clause id="sec-temporal-totaldurationnanoseconds" aoid="TotalDurationNanoseconds">
+      <h1>TotalDurationNanoseconds ( _days_, _hours_, _minutes_, _seconds_, _milliseconds_, _microseconds_, _nanoseconds_, _offsetShift_ )</h1>
+      <p>
+        The abstract operation TotalDurationNanoseconds computes a mathematical value number of nanoseconds from the given units, applying a given time zone offset shift in nanoseconds when converting from days to hours.
+      </p>
       <emu-alg>
+        1. If _days_ ≠ 0, then
+          1. Set _nanoseconds_ to ℝ(_nanoseconds_) − ℝ(_offsetShift_).
         1. Set _hours_ to ℝ(_hours_) + ℝ(_days_) × 24.
         1. Set _minutes_ to ℝ(_minutes_) + _hours_ × 60.
         1. Set _seconds_ to ℝ(_seconds_) + _minutes_ × 60.
         1. Set _milliseconds_ to ℝ(_milliseconds_) + _seconds_ × 1000.
         1. Set _microseconds_ to ℝ(_microseconds_) + _milliseconds_ × 1000.
-        1. Set _nanoseconds_ to ℝ(_nanoseconds_) + _microseconds_ × 1000.
+        1. Return ℝ(_nanoseconds_) + _microseconds_ × 1000.
+      </emu-alg>
+    </emu-clause>
+
+    <emu-clause id="sec-temporal-balanceduration" aoid="BalanceDuration">
+      <h1>BalanceDuration ( _days_, _hours_, _minutes_, _seconds_, _milliseconds_, _microseconds_, _nanoseconds_, _largestUnit_ )</h1>
+      <emu-alg>
+        1. Set _nanoseconds_ to ! TotalDurationNanoseconds(_days_, _hours_, _minutes_, _seconds_, _milliseconds_, _microseconds_, _nanoseconds_, 0).
         1. Set _days_, _hours_, _minutes_, _seconds_, _milliseconds_, and _microseconds_ to 0.
         1. If _nanoseconds_ &lt; 0, let _sign_ be −1; else, let _sign_ be 1.
         1. Set _nanoseconds_ to abs(_nanoseconds_).