Background: What are Sequence Units?

Measurement systems frequently employ multiple units in sequence to express a single magnitude.

Example sanctioned units:

• Length: foot-and-inch, meter-and-centimeter
• Mass: pound-and-ounce, kilogram-and-gram

Duration units are currently excluded from the list of sanctioned units:

• Time: hour-and-minute, minute-and-second, day-and-hour

In this presentation I will use "time units" and "duration units" interchangeably

Time Units in Amount vs Temporal.Duration

Developers can already use Temporal.Duration and Intl.DurationFormat:

const formatter = new Intl.DurationFormat("en", { style: "long" });
formatter.format({ hours: 2, minutes: 30 });

const duration = Temporal.Duration.from({ hours: 2, minutes: 30 });
formatter.format(duration);

What should happen if they try to use Amount and/or Intl Sequence Units?

const formatter = new Intl.NumberFormat("en", { style: "unit", unit: "hour-and-minute" });
formatter.format({ hour: 2, minute: 30 });

const amount = new Amount({ hour: 2, minute: 30 }, "hour-and-minute");
formatter.format(amount);

The Two Perspectives in TG2

In recent TG2 discussions (June & July 2026), two distinct architectural viewpoints emerged regarding proposal scoping and platform consistency:

• Position 1: Exclude Time Units
  Focus on guiding developers to purpose-built, domain-specific APIs.

• Position 2: Include Time Units
  Focus on alignment with CLDR data and ease of use with Amount.

Position 1: Why Exclude Time Units? (1/3)

1. Duration units have footguns

• day-and-hour crosses timezone / DST boundaries
• month-and-day depends on calendar month lengths

2. Durations are complicated to format

• Digital formatting styles (2:30:00)
• Flexible handling of zero-valued fields

Note: Domain-specific functionality impacts both large (month, day) and small (minute, second) duration units.

Position 1: Why Exclude Time Units? (2/3)

3. The Web Platform should promote the right way to do things

• We just spent 9 years designing Temporal
• We shouldn't add a worse way to represent and format time units

4. Adding duration unit formatting to Intl.NF will make developers expect formatting options there

• Six years ago, we explicitly decided to make a separate Intl.DurationFormat instead of overloading Intl.NumberFormat with duration formatting options and functionality
• If we add duration unit formatting only now, then the lack of duration-specific features in Intl.NumberFormat will be a pain point for years to come

Position 1: Why Exclude Time Units? (3/3)

5. Amount is already more general than Intl.NumberFormat

• Amount will allow arbitrary units, like jupiter-radius
• Intl will throw when formatting these non-sanctioned units
• So even if Amount supports time units, Intl.NF need not

6. Intl is already a subset of CLDR

• CLDR has over 200 units; Intl has 45, each independently motivated
• Intl is already sanctioning a narrow subset of sequence units
• ECMA need not add time sequence units just because they are in CLDR

Position 2: Why Include Time Units? (1/2)

1. Alignment with CLDR and Amount Conversion

• Sequence units and unit conversions in Amount are defined in terms of CLDR.
• There are usage preferences like "duration-media" that use time units.

2. Ergonomics

• If Amount.prototype.convertTo can produce minute-and-second objects, it is arbitrary and ergonomic friction if those valid Amount objects throw errors when passed to Intl.NumberFormat.

Position 2: Why Include Time Units? (2/2)

3. Flexibility

• Developers supporting arbitrary unit conversion (distance, mass, energy, etc.) should not need to write separate code for handling time units.
• While DurationFormat is ideal for dedicated duration handling, it should not be the only permitted approach enforced by artificial restrictions.

4. Not a Formatting Hazard

• Sequence formatting does not perform calendar or time math; it simply formats quantities together (like feet-and-inches or pounds-and-ounces).
• Formatting a valid sequence like hour-and-minute is fundamentally just list formatting with unit styles, which poses no inherent architectural danger.

Open Questions for Discussion

Intl Sequence Units:

1. Should Intl.NumberFormat include time units as sanctioned sequence units?

Amount:

1. Should Amount support single time units?
   • Example: new Amount(5, "hour")
2. Should Amount support sequence time units?
   • Example: new Amount({ hour: 2, minute: 30 }, "hour-and-minute")
3. Should Amount support time unit conversions?
   • Example: new Amount(5.5, "hour").convertTo("hour-and-minute")