Intl.DateTimeFormat error with formatRange

[cutterbl]

Consider the following

const formatter = new Intl.DateTimeFormat("en-US", {
    weekday: "short",
    year: "numeric",
    month: "2-digit",
    day: "2-digit"
  });

const myDate = "2024-02-14T16:00:00.000Z";
const myDate2 = "2024-02-15T16:00:00.000Z";

console.log(formatter.formatRange(new Date(myDate), new Date(myDate2)))

What You Currently See

Wed, 2/14/2024 – Thu, 2/15/2024

What Is Expected

Wed, 02/14/2024 – Thu, 02/15/2024

If you call just format() then the output is according to the definition (2-digit month), but when using formatRange() you get an incorrect month output (numeric month).

It does this in all current major browser versions (Chrome, Edge, FF and Safari)

[sffc]

Thanks for the report. I verified that this seems to be an issue in either ICU or in how engines call ICU.

The range pattern gets selected from here:

https://www.unicode.org/cldr/charts/44/by_type/date_&_time.gregorian.html#153a46c24508d0d4

The selected pattern is E, M/d/y – E, M/d/y. The spec then says that this pattern should be changed to match the field widths in the request:

If no match was found from the previous step, check what the closest match is in the fallback locale chain, as in availableFormats. That is, this allows for adjusting the string value field's width, including adjusting between "MMM" and "MMMM", and using different variants of the same field, such as 'v' and 'z'.

https://unicode.org/reports/tr35/tr35-dates.html#intervalFormats

So it should change M to MM, but it seems something is missing and it is not doing that transformation.

The root cause may be https://unicode-org.atlassian.net/browse/ICU-12076

CC @anba @FrankYFTang @Constellation for input on whether there is anything in the engines that could be causing this behavior or if it is indeed inside of ICU.

[anba]

Yes, this is caused by ICU. The spec actually allows this behaviour, though. FormatDateTimePattern selects the formatting options from rangeFormatOptions, which can have different options than what's stored in dateTimeFormat.