Is there a way to use localized units (months, week days) in a more specific datetime formatting?

[acid-bong]

Goal: i'm writing a statusbar, and one of its blocks is supposed to update datetime every second.

Draft code:

// dependencies are sys-locale, icu_{locid,calendar,datetime}, jiff and jiff-icu
use jiff_icu::ConvertFrom; // this trait can convert jiff DateTime to icu DateTime 

fn main() {
    let locale: icu_locid::Locale = sys_locale::get_locale()
        .unwrap_or("en-US".to_string())
        .parse()
        .unwrap();

    let formatter =
        icu_datetime::TypedDateTimeFormatter::try_new(&locale.into(), Default::default()).unwrap();

    loop {
        let now = jiff::Zoned::now().datetime();
        let now_icu =
            icu_calendar::DateTime::convert_from(now).to_calendar(icu_calendar::Gregorian);
        let now_fmt = formatter.format_to_string(&now_icu);
        println!("{}", &now_fmt);
        std::thread::sleep(std::time::Duration::from_secs(1));
    }
}

Expected functionality: means to format the date using localized values (strftime style), so it looks like %a, %d %b %Y :: %T -> Вс, 23 фев 2025 :: 18:47:05, akin to Python's time.strftime(format) or GNU coreutils' date +FORMAT, which do follow a given locale.

Actual functionality: [Typed]DateTimeFormatter isn't able to format datetime after a pattern, and jiff's strftime can't use localized units from icu (yet?). Values in icu_datetime::options::length don't seem to give full control over the output.

Is there a different way I can achieve that?

[sffc]

The best i18n behavior is almost always to use the locale-provided patterns. You shouldn't expect that the year/month/day are always in a particular order, or that the month is always numeric, etc.

With that disclaimer out of the way, you can do what you are asking for with:

• Tip of main and 2.0.0-beta2: icu::datetime::pattern::FixedCalendarDateTimeNames
  • https://unicode-org.github.io/icu4x/rustdoc/icu/datetime/pattern/struct.FixedCalendarDateTimeNames.html
• 2.0.0-beta1: icu::datetime::pattern::TypedDateTimeNames
  • https://docs.rs/icu/2.0.0-beta1/icu/datetime/pattern/struct.DateTimePattern.html
• 1.5.0: icu::datetime::TypedDateTimeNames
  • https://docs.rs/icu/1.5.0/icu/datetime/struct.TypedDateTimeNames.html

Sorry for this not being discoverable on 1.5; it is improved on the 2.0 branch.

[acid-bong]

Hmm, looks quite complicated for what I expected to be a basically oneliner. Looks like I have to use a custom pattern, since none of the default ones include days of the week and shortened months together

[acid-bong]

I guess i can leave day/month/year to the locale, but can I prepend and append additional stuff to the existing format, like 'date icon' E, date 'time icon' time?

[robertbastian]

This is with 2.0.0-beta2

use icu::datetime::{
    fieldsets,
    input::{Date, DateTime, Time},
    DateTimeFormatter, NoCalendarFormatter,
};
use icu::locale::locale;

fn main() {
    let prefs = locale!("fr").into();

    let ymd = DateTimeFormatter::try_new(prefs, fieldsets::YMDE::medium()).unwrap();
    let time = NoCalendarFormatter::try_new(prefs, fieldsets::T::short()).unwrap();

    loop {
        let now = jiff::Zoned::now().datetime();

        // jiff-icu doesn't support 2.0 yet
        let datetime = DateTime {
            date: Date::try_new_gregorian(
                i32::from(now.year()),
                now.month().unsigned_abs(),
                now.day().unsigned_abs(),
            )
            .unwrap(),
            time: Time::try_new(
                now.hour().unsigned_abs(),
                now.minute().unsigned_abs(),
                now.second().unsigned_abs(),
                now.subsec_nanosecond().unsigned_abs(),
            )
            .unwrap(),
        };

        println!(
            "{} :: {}",
            ymd.format(&datetime),
            time.format(&datetime.time)
        ); // jeu. 27 févr. 2025 :: 21:01:17

        std::thread::sleep(core::time::Duration::from_secs(1));
    }
}

[sffc]

@robertbastian's example looks good if you wish to manually interpolate the date and time together into a pattern. Most users wishing to use datetime formatting though should prefer to use fieldsets::YMDET, which uses a locale-specific "glue pattern" for the date and time.

[acid-bong]

That example looks epic, I'll try it out. However, I'd rather use YMDE and T separately, as I wanna insert a coloured icon between the date and the time.

Thanks for the tips