PartitionDateTimeRangePattern may need additional date-time fields not present in the formatter

[anba]

Example:

js> var dtf = new Intl.DateTimeFormat("en", {hour: "numeric", timeZone: "UTC"})
js> dtf.resolvedOptions()
({locale:"en", calendar:"gregory", numberingSystem:"latn", timeZone:"UTC", hourCycle:"h12", hour12:true, hour:"numeric"})
js> dtf.formatRange(0, 24*60*60*1000)                                                               
"1/1/1970, 12 AM \u2013 1/2/1970, 12 AM"

The resolved pattern for the Intl.DateTimeFormatter only includes [[hour]], but to output the range more date-time fields are needed.

---

This can mean two things:

1. Either restrict [[rangePatterns]] to only include fields present in the format fields record.
   1. For example the format fields record {[[hour]]: "numeric", [[minute]]: "2-digit", [[second]]: "2-digit", [[pattern]]: "{hour}:{minute}:{second}", [[pattern12]]: "{hour}:{minute}:{second} {ampm}"} can only support ranges in [[rangePatterns]] which contain hour, minute, and second.
   2. This mode isn't currently supported by ICU, though.
2. Somehow create a separate Intl.DateTimeFormat instance in PartitionDateTimeRangePattern which supports all fields needed for [[rangePatterns]].

[anba]

Or instead of a whole new separate Intl.DateTimeFormat instance, give the [[rangePatterns]] elements additional fields to define the date-time formats used within the ranges.

For example instead of the current:

[[rangePatterns]]:
    [[Hour]]:
        {[[Source]]: "startRange", [[Pattern]]: "{day}/{month}/{year}, {hour}:{minute}"}
        {[[Source]]: "shared", [[Pattern]]: " – "}
        {[[Source]]: "endRange", [[Pattern]]: "{hour}:{minute}"}
[...]

define it as

[[rangePatterns]]:
    [[Hour]]:
        [[day]]: "numeric"
        [[month]]: "numeric"
        [[year]]: "numeric"
        [[hour]]: "numeric"
        [[minute]]: "2-digit"
        {[[Source]]: "startRange", [[Pattern]]: "{day}/{month}/{year}, {hour}:{minute}"}
        {[[Source]]: "shared", [[Pattern]]: " – "}
        {[[Source]]: "endRange", [[Pattern]]: "{hour}:{minute}"}
[...]

and then use the additional fields within FormatDateTimePattern.

[anba]

Hmm, ICU-based implementations can't even guarantee that the interval and non-interval string use the same output format for date-time fields:

js> var dtf = new Intl.DateTimeFormat("en", {day: "2-digit", timeZone: "UTC"})  
js> dtf.format(0)                                                   
"01"
js> dtf.formatRange(0, 24*60*60*1000)                                          
"1 \u2013 2"

2-digit is only used for the non-interval string, whereas the interval string uses numeric.

So, if we want to specify what's currently implementable using ICU, we need to allow to use different formats for the individual date-time components, which is possible with the proposal outlined in #21 (comment).

[anba]

@FrankYFTang Was your comment meant for #22?

[FrankYFTang]

@FrankYFTang Was your comment meant for #22?

yes, sorry. I put into the wrong issue. Deleted that and added it to #22

[fabalbon]

Thanks @anba for the suggestion, you are right that we are missing these date-time fields that would be necesary to format some of the possible date ranges.

I like your proposal as it would allow us to support current ICU behavior without needing to create a new dateTimeFormat in PartitionDateTimeRangePattern. I prepared #25 based on it.

@anba, @FrankYFTang I'd really appreciate if you could take a look at and review #25. Thanks!

[anba]

A Firefox user also noticed that the date-time field formats can change depending on the input ranges: https://bugzilla.mozilla.org/show_bug.cgi?id=1659367