Support Intl.RelativeTimeFormat for `relativeTime` formatting

[kvnxiao]

Is your feature request related to a problem? Please describe.

Currently, the formatter for Formatting date and time ranges does not support additional configuration other than specifying the exact unit to display the output in.

The relative time formatting should support all the options from Intl.RelativeTimeFormat and not just RelativeTimeFormatUnit

Describe the solution you'd like

Instead of:

type RelativeTimeFormatOptions = {
    now?: number | Date;
    unit?: Intl.RelativeTimeFormatUnit;
};
export default RelativeTimeFormatOptions;

The RelativeTimeFormatOptions type should also accept the options from the Intl.RelativeTimeFormat constructor:

• numberingSystem
• style
• numeric

Describe alternatives you've considered

The only alternative would be to use the native Intl API directly, but this should be integrated into the formatter in next-intl IMO

[amannn]

Hey, thanks for the report! I agree, we could definitely support numberingSystem and style.

As for numeric I'm a bit hesitant though, as it can lead to issues when values are rounded: #765.

See also:

next-intl/packages/use-intl/src/core/createFormatter.tsx

Lines 247 to 257 in 3eefe6f

	return new Intl.RelativeTimeFormat(locale, {
	// `numeric: 'auto'` can theoretically produce output like "yesterday",
	// but it only works with integers. E.g. -1 day will produce "yesterday",
	// but -1.1 days will produce "-1.1 days". Rounding before formatting is
	// not desired, as the given dates might cross a threshold were the
	// output isn't correct anymore. Example: 2024-01-08T23:00:00.000Z and
	// 2024-01-08T01:00:00.000Z would produce "yesterday", which is not the
	// case. By using `always` we can ensure correct output. The only exception
	// is the formatting of times <1 second as "now".
	numeric: unit === 'second' ? 'auto' : 'always'
	}).format(value, unit);

Do you need numeric for your use case? If so, can you include further details?

I've added #1057 to look into this.

[kvnxiao]

At the moment, I only have a need for the style to specify long, short, or narrow.

I think numberingSystem and style is sufficient for now!

[amannn]

Sounds good, thanks for the feedback!

[kvnxiao]

Circling back to this @amannn I have come across needing the numeric formatting option now for the formatting style to show "today" and "yesterday" 😅

[amannn]

Interesting. Can you share what your exact use case is?

From my personal observation, "yesterday" is a nice thing to read, but it's easy to run into a wrong output due to rounding (see #1056 (comment)). "Yesterday" might not actually be the day before today, but could be the day before yesterday.

[kvnxiao]

The use case is exactly as described, requiring the use of localizations for "yesterday" and "today". I don't think we should be too worried about the rounding because IMO the idea is that doing the correct math beforehand is a prerequisite that's on the onus of the developer (otherwise it wouldn't make much sense for the official Intl.RelativeTimeFormat to support numeric: "auto")

For what it's worth, what I'm currently doing is I'm using Luxon to do proper date-time calculations of the math in "days" (with the local machine timezones set) via dateTime.startOf('day'), before converting it back into a JS Date and passing it into the Intl.RelativeTimeFormat with numeric: auto.

[kvnxiao]

Let me know if you would like me to open a new issue to track this discussion!

[amannn]

The use case is exactly as described, requiring the use of localizations for "yesterday" and "today"

The current output would be "1 day ago" instead of "yesterday". I'd say that "yesterday" reads a bit nicer, but the rounding problem make this a bit error prone. Is there a particular issue with using "1 day ago" in your app?

otherwise it wouldn't make much sense for the official Intl.RelativeTimeFormat to support numeric: "auto"

So with auto, you either get values like "yesterday" (for integers) or "1.1 days ago" (for floating point numbers). That's when you're using Intl.RelativeTimeFormat yourself—without next-intl.

However, next-intl does rounding internally in order to render more readable values like "1 day ago"—an approximation.

next-intl/packages/use-intl/src/core/createFormatter.tsx

Lines 69 to 71 in 9a70cef

	// We have to round the resulting values, as `Intl.RelativeTimeFormat`
	// will include fractions like '2.1 hours ago'.
	return Math.round(seconds / UNIT_SECONDS[unit]);

The docs mention this capability:

Note that values are rounded, so e.g. if 126 minutes have passed, “2 hours ago” will be returned.

So if next-intl would allow to pass numeric: 'auto' we'd have the user run into problematic scenarios due to rounding. If you're using Intl.RelativeTimeFormat directly, the rounding is on you. It seems to me like that puts you in a better position than if next-intl would support this.

That's at least my current understanding. I might have to give this more thought, but it seems like a potentially problematic feature to support.

What do you think?

[kvnxiao]

Is there a particular issue with using "1 day ago" in your app

Yes, the designers specifically want more friendly localized dates for "today" and "yesterday", instead of "1 day ago".

Currently I'm fetching the locale via next-intl's useLocale and passing it into Intl.RelativeTimeFormat directly. May I ask, what's wrong with trying to support the same feature set as Intl.RelativeTimeFormat directly? The onus is already on the developer to make sure they're doing the rounding properly when using Intl.RelativeTimeFormat and numeric: auto, so can we not just document that as a caveat and mirror it's behaviour in next-intl directly? This is just a minor inconvenience, but if next-intl had this support directly in the formatter it would just save me having to pass around the locale to various Intl.XYZ functions.