Add locale-aware date formatting for UI dates

[rodrigost23]

Description

Adds locale-aware dates, fixes #470.

Demo

Checklist

• I have read CONTRIBUTING.md in its entirety
• I have performed a self-review of my own code
• I have added unit tests to cover my changes
• The last commit successfully passed pre-commit checks
• Any AI code was thoroughly reviewed by me

[krokosik]

I suppose we could just use new Intl.DateTimeFormat(todayTranslation.usedLng, {day: '2-digit', month: 'short'}).format(date)?

[rodrigost23]

I tried that, but this Intl API is kind of weird. At least for pt-BR and pt-PT, the results are awful, so I'm not sure what other languages would look like to native speakers.

pt-PT:

I don't know why it would format "short" month to "11" (we have "numeric" for that, right?). 

pt-BR:

The word "de" here is just a preposition that we can totally ignore when we want it to be shorter.

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Maybe we could have a specific function only for this expense date prefix and use the Intl for other places, which would probably make this look better:
 

[krokosik]

I'm not a native speaker in either of these, but maybe it is simply the proper way to format this in Portugal? For the brazilian version, yeah it looks kind of bad.

Personally I would prefer fixing locale specific edge cases rather than to import the bulky date-fns locale and pass it around.

For the expense list, we could format the day and month separately and remove dots for now?

new Intl.DateTimeFormat('pt-PT', {month: 'short'}).format(new Date()).replace('.', '')

Actually returns the month name, so we could just built this in a consistent format. What do you think?

[rodrigost23]

Formatting day and month separately makes sense.

Do we need to care about the order, though? Some locales use MM/DD, others DD/MM.

There’s a way to detect that order if we really want to (expand for an example)

function formatDayMonth(locale: string, date: Date): string { // Format day and short month separately const day = new Intl.DateTimeFormat(locale, { day: 'numeric' }).format(date); const monthName = new Intl.DateTimeFormat(locale, { month: 'short' }).format(date) .replace('.', ''); // Detect whether day comes before month for this locale const orderParts = new Intl.DateTimeFormat(locale, { day: 'numeric', month: 'numeric' }).formatToParts(date); const dayIndex = orderParts.findIndex(p => p.type === 'day'); const monthIndex = orderParts.findIndex(p => p.type === 'month'); const isDayFirst = dayIndex < monthIndex; return isDayFirst ? `${day} ${monthName}` : `${monthName} ${day}`; } // Examples: formatDayMonth('pt-BR', new Date('2025-10-11')); // "11 out" formatDayMonth('pt-PT', new Date('2025-10-11')); // "11 out" formatDayMonth('de', new Date('2025-10-11')); // "11 Okt" formatDayMonth('en-GB', new Date('2025-10-11')) // "11 Oct" formatDayMonth('en-US', new Date('2025-10-11')); // "Oct 11" formatDayMonth('en', new Date('2025-10-11')); // "Oct 11" formatDayMonth('zh', new Date('2025-10-11')) // "10月 11日" Feels a bit over-engineered, but it works. In pt-BR I wouldn't mind if it stayed as "MM DD", since the layout stacks them on top of each other in the expense list. Not sure how that reads in other locales...

What do you think? We just keep "MM DD" specifically in the expenses list for now? In that case, the following should be enough:

const day = new Intl.DateTimeFormat(todayTranslation.usedLng, { day: 'numeric' }).format(date);
const monthName = new Intl.DateTimeFormat(todayTranslation.usedLng, { month: 'short' }).format(date).replace('.', '');

return `${monthName} ${day}`;

[krokosik]

Looks good to me. One change would be to change day to 2-digit maybe?