Negative durations

Changes Temporal.Duration to allow negative values in the fields, as
long as all the fields have the same sign. Adds negated() and abs()
methods to Temporal.Duration, and a sign property.

On all types with arithmetic, subtracting a negative duration is
equivalent to adding the absolute value of that duration, and adding a
negative duration is equivalent to subtracting the absolute value of
that duration.

Closes: #782

docs/absolute.md

@@ -254,6 +254,8 @@ If you need to do this, convert the `Temporal.Absolute` to a `Temporal.DateTime`
 
 If the result is earlier or later than the range that `Temporal.Absolute` can represent (approximately half a million years centered on the [Unix epoch](https://en.wikipedia.org/wiki/Unix_time)), a `RangeError` will be thrown.
 
+Adding a negative duration is equivalent to subtracting the absolute value of that duration.
+
 Example usage:
 ```js
 // Temporal.Absolute representing five hours from now
@@ -279,6 +281,8 @@ If you need to do this, convert the `Temporal.Absolute` to a `Temporal.DateTime`
 
 If the result is earlier or later than the range that `Temporal.Absolute` can represent (approximately half a million years centered on the [Unix epoch](https://en.wikipedia.org/wiki/Unix_time)), a `RangeError` will be thrown.
 
+Subtracting a negative duration is equivalent to adding the absolute value of that duration.
+
 Example usage:
 ```js
 // Temporal.Absolute representing this time yesterday

docs/balancing.md

@@ -27,34 +27,33 @@ Therefore, any `Duration` object with nonzero years or months can refer to a dif
 No balancing is ever performed between years, months, and days, because such conversion would be ambiguous.
 If you need such a conversion, you must implement it yourself, since the rules can depend on the start date and the calendar in use.
 
-Negative values are never allowed as `Temporal.Duration` fields, so passing one as an argument to `new Temporal.Duration()` or as a property in the object passed to `Temporal.Duration.from()` will always throw an exception, regardless of the disambiguation mode.
+`Temporal.Duration` fields are not allowed to have mixed signs, so, for example, passing one positive and one negative argument to `new Temporal.Duration()` or as a properties in the object passed to `Temporal.Duration.from()` will always throw an exception, regardless of the disambiguation mode.
 
 Therefore, the only case where constrain and reject mode have any effect when creating a duration, is integer overflow.
-If one of the values overflows, constrain mode will cap it to `Number.MAX_VALUE`.
+If one of the values overflows, constrain mode will cap it to `Number.MAX_VALUE` or `-Number.MAX_VALUE`.
 
 ## Duration arithmetic
 
-When adding two `Temporal.Duration`s, the situation is much the same as for constructing one.
-It's not possible to end up with a negative value when adding two valid durations, nor is any balancing needed, so the disambiguation mode only determines what to do in the case of integer overflow.
+When adding or subtracting two `Temporal.Duration`s, the situation is different.
+As well as the same possibility of integer overflow, the fields may also need to be balanced.
 
-The situation is very different for subtraction.
 Consider a duration of 90 minutes, from which is subtracted 30 seconds.
 Subtracting the fields directly would result in 90 minutes and −30 seconds, which is invalid.
 Therefore, it's necessary to balance the seconds with the minutes, resulting in 89 minutes and 30 seconds.
 
-By default, the fields of the resulting duration are only converted between each other if one unit is negative but a larger unit is positive, in which case the smaller is balanced with the larger to avoid having negative-valued fields.
+By default, the fields of the resulting duration are only converted between each other if one unit is negative but a larger unit is positive, or vice versa, in which case the smaller is balanced with the larger to avoid having mixed-sign fields.
 
 That's not all the balancing that _could_ be done on the resulting value.
 It could further be balanced into 1 hour, 29 minutes, and 30 seconds.
 However, that would likely conflict with the intention of having a duration of 90 minutes in the first place, so this should be behaviour that the Temporal user opts in to.
-With subtraction, we make a distinction between "necessary balancing" and "optional balancing".
+Here, we make a distinction between "necessary balancing" and "optional balancing".
 
-In order to accommodate this, the `disambiguation` option when subtracting `Temporal.Duration`s is different from all the other arithmetic methods' disambiguation options.
-Necessary balancing is called `balanceConstrain` mode, because values are constrained to be non-negative through balancing.
+In order to accommodate this, the `disambiguation` option when performing arithmetic on `Temporal.Duration`s is different from all the other arithmetic methods' disambiguation options.
+Necessary balancing is called `constrain` mode, because values are constrained to be non-negative through balancing.
 Optional balancing is called `balance` mode.
-The usual `constrain` and `reject` modes are not available.
+The usual `reject` mode is also available, and does the same thing as `constrain` but throws on integer overflow.
 
-The default is `balanceConstrain` mode.
+The default is `constrain` mode.
 The `balance` mode is only provided for convenience, since the following code snippets give the same result:
 
 ```javascript
@@ -95,8 +94,8 @@ one.minus(two, { disambiguation: 'balance' });  // => PT1H45M
 // Result is negative
 one = Temporal.Duration.from({ hours: 2, minutes: 30 });
 two = Temporal.Duration.from({ hours: 3 });
-one.minus(two);                                 // throws
-one.minus(two, { disambiguation: 'balance' });  // throws
+one.minus(two);                                 // -PT30M
+one.minus(two, { disambiguation: 'balance' });  // -PT30M
 
 // Unbalanceable units, but also no balancing possible
 one = Temporal.Duration.from({ months: 3, days: 15 });

docs/date.md

@@ -323,6 +323,8 @@ For these cases, the `disambiguation` option tells what to do:
 
 Additionally, if the result is earlier or later than the range of dates that `Temporal.Date` can represent (approximately half a million years centered on the [Unix epoch](https://en.wikipedia.org/wiki/Unix_time)), then this method will throw a `RangeError` regardless of `disambiguation`.
 
+Adding a negative duration is equivalent to subtracting the absolute value of that duration.
+
 Usage example:
 ```javascript
 date = Temporal.Date.from('2006-08-24');
@@ -357,6 +359,8 @@ For these cases, the `disambiguation` option tells what to do:
 
 Additionally, if the result is earlier or later than the range of dates that `Temporal.Date` can represent (approximately half a million years centered on the [Unix epoch](https://en.wikipedia.org/wiki/Unix_time)), then this method will throw a `RangeError` regardless of `disambiguation`.
 
+Subtracting a negative duration is equivalent to adding the absolute value of that duration.
+
 Usage example:
 ```javascript
 date = Temporal.Date.from('2006-08-24');

docs/datetime.md

@@ -387,6 +387,8 @@ For these cases, the `disambiguation` option tells what to do:
 
 Additionally, if the result is earlier or later than the range of dates that `Temporal.DateTime` can represent (approximately half a million years centered on the [Unix epoch](https://en.wikipedia.org/wiki/Unix_time)), then this method will throw a `RangeError` regardless of `disambiguation`.
 
+Adding a negative duration is equivalent to subtracting the absolute value of that duration.
+
 Usage example:
 ```javascript
 dt = new Temporal.DateTime(1995, 12, 7, 3, 24, 30, 0, 3, 500);
@@ -421,6 +423,8 @@ For these cases, the `disambiguation` option tells what to do:
 
 Additionally, if the result is earlier or later than the range of dates that `Temporal.DateTime` can represent (approximately half a million years centered on the [Unix epoch](https://en.wikipedia.org/wiki/Unix_time)), then this method will throw a `RangeError` regardless of `disambiguation`.
 
+Subtracting a negative duration is equivalent to adding the absolute value of that duration.
+
 Usage example:
 ```javascript
 dt = new Temporal.DateTime(1995, 12, 7, 3, 24, 30, 0, 3, 500);