Duration / time units in Amount

[sffc]

Related: #118, tc39/proposal-intl-sequence-units#8

units.xml defines a bunch of duration units and some conversions between them:

        <!-- time -->
        <convertUnit source='second' baseUnit='second' systems="si metric prefixable ussystem uksystem astronomical"/>
        <convertUnit source='minute' baseUnit='second' factor='60' systems="si_acceptable metric_adjacent prefixable ussystem uksystem astronomical"/>
        <convertUnit source='hour' baseUnit='second' factor='3600' systems="si_acceptable metric_adjacent prefixable ussystem uksystem astronomical"/>
        <convertUnit source='day' baseUnit='second' factor='86400' systems="si_acceptable metric_adjacent prefixable ussystem uksystem astronomical"/>
        <convertUnit source='day-person' baseUnit='second' factor='86400' systems="person_age"/>
        <convertUnit source='week' baseUnit='second' factor='86400*7' systems="metric_adjacent ussystem uksystem astronomical"/>
        <convertUnit source='week-person' baseUnit='second' factor='86400*7' systems="person_age"/>
        <convertUnit source='fortnight' baseUnit='second' factor='86400*14' systems="ussystem uksystem"/>
        
        <!-- year-duration -->
        <convertUnit source='month' baseUnit='year' factor='1/12' systems="metric_adjacent ussystem uksystem astronomical"/>
        <convertUnit source='month-person' baseUnit='year' factor='1/12' systems="person_age"/>
        <convertUnit source='quarter' baseUnit='year' factor='1/4' systems="metric_adjacent"/>
        <convertUnit source='year' baseUnit='year' systems="metric_adjacent prefixable ussystem uksystem astronomical"/>
        <convertUnit source='year-person' baseUnit='year' systems="person_age"/>
        <convertUnit source='decade' baseUnit='year' factor='10' systems="metric_adjacent ussystem uksystem astronomical"/>
        <convertUnit source='century' baseUnit='year' factor='100' systems="metric_adjacent ussystem uksystem astronomical"/>
        
        <!-- night-duration -->
        <convertUnit source='night' baseUnit='night' systems="metric_adjacent uksystem ussystem"/>     

However, we have Temporal.Duration already in JavaScript for handling duration units and conversions, which was designed to better handle the edge cases involved with duration units; see this presentation for details.

CLDR is a baseline, but TC39 should take ECMAScript-specific considerations into account, not just blindly take everything that is in units.xml.

Some questions:

• Should we support time/duration units at all? If so, which ones?
• Should we support conversions between time/duration units? If so, which ones?
• Should we support time/duration units in sequence units? If so, which ones?

[eemeli]

Should we support time/duration units at all? If so, which ones?

The only place in the Amount spec where we actually care about the units is when doing unit conversions (which you ask about separately); formatting is delegated to Intl.NumberFormat, which has some separate opinions. So the answer to this question must be "yes, all of them".

Should we support conversions between time/duration units? If so, which ones?

Yes, all conversions within each category should be supported. You mention the time and year-duration unit categories, but we should also support conversions within the following unit categories, which also depend on time units:

• acceleration
• catalytic-activity
• electric-capacitance
• electric-conductance
• electric-charge
• electric-inductance
• electric-resistance
• energy
• force
• frequency
• ionizing-radiation
• magnetic-induction
• power
• pressure
• pressure-per-length
• radioactivity
• speed
• voltage

Should we support time/duration units in sequence units? If so, which ones?

The same as for all other units: For construction, all of them. For conversion, if they belong within the same category. For formatting, whatever Intl.NumberFormat supports.

[sffc]

My opinion:

ECMAScript includes a good way to perform time unit representation and conversion. Anything we do in Amount will be worse than what we do in Temporal. We should therefore not do more than we absolutely must do in Amount.

ECMAScript should make its own decisions about which conversions to support. Having something in CLDR is a prerequisite for inclusion, but it does not imply inclusion.

I think Amount time/duration unit conversion should forbid conversions between units that require a relativeTo parameter in Temporal.Duration.prototype.round. In practice, this means no conversions between years, months, and days. It appears the Temporal champions decided to allow day-to-hour; I probably would have done the more conservative option of not allowing this, but it seems reasonable to follow TC39 precedent here.

CLDR already forbids day-to-month, but it allows month-to-year. The only real deviation between Amount and CLDR I would like to see, therefore, is that we should put month, month-person, and quarter in their own unit category.

As far as sequence units, I am in favor of only allowing sanctioned sequence units, by some definition of "sanctioned", probably those that support conversion. Then, the units allowed to be in sequence units are those that we allow conversion between.